PHOTOS: Federer, Nadal advance

Roger Federer avenged his loss to Jo-Wilfried Tsonga at Wimbledon this year when he beat the Frenchman 6-4, 6-3, 6-3 on Thursday to reach the semi-finals of the US Open and give himself a chance of a sixth title at Flushing Meadows.

Showing no signs of slowing down in his first Grand Slam tournament as a 30-year-old, Federer used his serve as a lethal weapon, whipped angled winners with his groundstrokes and played superbly at the net.

The third-seeded Federer advanced to a scintillating showdown in the last four against top-seeded Novak Djokovic and his eighth straight US Open semi.

World No 1 Djokovic, the Australian and Wimbledon champion, reached the semis when his Serbian Davis Cup team mate Janko Tipsarevic retired with a leg injury while trailing 7-6, 6-7, 6-0, 3-0.

"I played great," Federer said in an on-court interview. "For me it continues, and that is awesome.

"I'm very happy with the way I'm playing, the way I'm moving and I'm enjoying myself on the court."

Top seed Novak Djokovic advanced to the semi-finals of the US Open on Thursday when his Serbian Davis Cup team mate Janko Tipsarevic retired in the fourth set of their quarter-final match.

Djokovic had been leading 7-6, 6-7, 6-0, 3-0 when Tipsarevic, whose leg was strapped during the match, quit after three hours, 27 minutes of play at Arthur Ashe Stadium after feeling sharp pain in his left leg.

"The fact is that it's a quarter-finals and he's first time in his career in the quarter-finals of a Grand Slam. He really put a great effort out there," Djokovic said.

"And then physically, unfortunately, he couldn't hold on. You could see that he could not move as well as he did in (the first) two and a half hours."

Tipsarevic said he might have tried to push on through the discomfort were it not for defending champion Serbia's Davis Cup semi-final next weekend against Argentina.

"You know, that's his decision. I cannot comment on that," Djokovic said.

"I support anything he does because he's my friend and I know him well, and I know he's somebody who doesn't like to retire.

"Definitely he's a big fighter, and that just says how much he respects his country and how much he appreciates to play for his country. He wants to be fit."

Australian and Wimbledon champion Djokovic won the first-set tie-break 7-2, and lost the second 7-3 before surging ahead as his compatriot began slowing down.

The World No. 1 also gave onlookers a scare when he doubled over and called for the trainer after making a spectacular sliding forehand winner in the first game of the fourth set.

After getting the big toe on his left foot treated and bandaged, Djokovic went back to business before the 20th-seeded Tipsarevic signaled he had had enough.

"My left toe is bleeding," explained Djokovic. "These things happen all the time when you're sliding like I do."

World No. 4 Andy Murray is already familiar with the added burden of expectation as he has long carried the hopes of a British nation counting on him to snap a 75-year Grand Slam drought.

Murray, vying to become Britain's first Grand Slam champion since Fred Perry won the U.S. Open in 1936, may have his best shot yet but his quest now carries an added challenge.

After two days of persistent rain halted play at the last Grand Slam of the year, Murray now faces a physical and mental test of having to possibly play four matches in five days before he can be crowned champion.

He will only get a day off if he makes the final, which tournament organisers earlier decided would be played on Monday instead of the originally planned Sunday.

Murray came through the first of those four matches in great style on Thursday, thumping American wildcard Donald Young 6-2, 6-3, 6-3 in under two hours.

It was sweet revenge for the Scotsman after he was beaten by Young in their only previous meeting, earlier this year, when he was struggling with his form and confidence.

"It was a tough moment for me," Murray said. "Obviously, not only are you playing badly, but you're not happy either when you're playing like that."

Things have gone much better for Murray since then and he is reaching his peak at the right time at one of his favourite tournaments.

He won a lead-up event in Cincinnati and has been steadily improving with each match at Flushing Meadows but knows the toughest challenges are still ahead of him.

Murray's next opponent is John Isner, the towering American with one of the most lethal serves in the game.

"He's got a huge serve and can hit big forehands and is a little bit erratic and doesn't give you much rhythm," Murray said.

Isner, seeded 28th, reached the quarter-finals for the first time with a 7-6, 3-6, 7-6, 7-6 upset win over Frenchman Gilles Simon but was expecting a tougher match from Murray.

"I played him in the Australia Open in 2010 and he beat me pretty easily," Isner said. "He would kind of take the air out of the ball and hit a lot of short angles and got me moving.

"I'm going to have to change a couple things up. He's one of the best returners in the game, so I'm going to have to serve my best. When I do get my chances on the return, I will have to take them."

Former champion Andy Roddick fumed over another snafu regarding court conditions on Thursday before shifting to another court to book his place in the US Open semi-finals.

Roddick, the 2003 US Open winner, beat fifth-seeded Spaniard David Ferrer 6-3, 6-4, 3-6, 6-3, completing the quarter-final on distant Court 13 after a wet spot behind the baseline on Louis Armstrong Stadium court could not be fixed.

When they called Roddick and Ferrer back to the court, after using a vacuum cleaner and thin rubber hose and towels to try and fix the problem, the American was angered to see water still bubbling up to the surface.

"I'm baffled, I'm baffled. Why are we out here?" he railed at tournament referee Brian Earley. "Find us a court."

Roddick went back into the hallway and continued to vent his frustration at not getting on with his match, given the compressed schedule forced by two successive washouts at Flushing Meadows.

When the players were asked whether they would be willing to move to Court 13 rather than wait for a chance to get on the grandstand showcourt, Roddick said: "Let's go play, let's go play. We just want to play."

On Wednesday, Roddick, Rafa Nadal and Andy Murray together lodged their complaints about being asked to compete in damp conditions to tournament officials before play was halted for the day.

Once resettled on Thursday at Court 13 after a delay of about one hour, Roddick played some of his best tennis to set up a last four showdown with Spaniard Nadal.

It was a rare trip outside the confines of the main courts for Roddick, who as the leading U.S. player for most of the last decade has exclusively played on the showcourts.

"I enjoyed it," said the 29-year-old Roddick. "I like playing kind of the smaller, more intimate stuff when I can.

"It was a little bit of everything. We had some Van Morrison wannabe playing music in the courtyard...there was a guy scaling the fence in the back for a second...(and) a couple of people wanted to do commentary from the service line.

"There was a repetitive screaming from the courtyard at one point. It was actually kind of shrill. It was a little stressful. It sounded like someone was getting hurt. So I don't know if that's what it's always like out there."

Defending champion Rafa Nadal welcomed the return of sunshine to Flushing Meadows on Thursday by completing a resounding 7-6, 6-1, 6-2 victory over Gilles Muller of Luxembourg to reach the US Open quarter-finals.

Nadal trailed 3-0 Wednesday after 16 minutes of play and then complained about being asked to go on court in what he considered wet conditions before the entire day's program was called off.

The Spaniard wasted no time exerting his superiority Thursday, getting the first set back on serve and claiming the tiebreaker 7-1 before racing through the last two sets.

"Yesterday was a tough day for everybody," Nadal told the crowd at Arthur Ashe Stadium. "Sorry, but thank you very much for still being here."

The second-seeded Nadal will play former champion Andy Roddick.

Serena Williams struggled with her serve in the first set before finding the range to score a 7-5, 6-1 victory over Anastasia Pavlyuchenkova of Russia on Thursday to reach the semi-finals of the US Open.

Three-time winner Williams lost serve three times in the opening set as the first six games went against serve before the American got on track, breaking the 17th-seeded Russian in the 12th game to end the opening set.

"It was a slow start, and once I lost serve, I was determined to break serve so I wouldn't be down a break," said Williams. "Then I just kept losing serve in the beginning. Very weird."

The 29-year-old Williams will play Wozniacki in the semi-finals on Saturday.

Top-seed Caroline Wozniacki defeated German 10th seed Andrea Petkovic 6-1, 7-6 on Thursday to reach the semi-finals of the US Open.

World No 1 Wozniacki, aiming to claim her first Grand Slam title, squandered a 5-3 lead serving for the second set but broke Petkovic in the 12th game to force a tiebreaker, which she won 7-5.

Wozniacki, coming off a three-set struggle against former US Open winner Svetlana Kuznetsova in the fourth round, was shifted to Court 13 for the quarter-final after a wet spot on the Louis Armstrong Stadium court could not be fixed.

The 21-year-old Dane was expecting to face another stern test in the last four, where she will meet three-time US Open winner Serena Williams, who beat 17th-seeded Russian Anastasia Pavlyuchenkova 7-5, 6-1.

"I'm looking forward to that match," Wozniacki said in an on-court interview about the 29-year-old Williams, who has been sidelined most of the year due to injuries and illness.

"She's an amazing athlete and a great girl. She's a fighter just like I am on the court. I got to go out there and take my chances."

Australia's long wait for a female Grand Slam champion could soon be over after Sam Stosur's wild ride through the US Open took her to the semi-finals.

On Thursday, she demolished the world number two and last year's runner-up Vera Zvonareva 6-3, 6-3, in 67 minutes with one of the most impressive performances of the tournament.

She never once faced break point on her serve and thumped 25 winners past her hapless opponent who may have been ranked eight places higher, but had lost each of her last seven matches against the Queenslander.

"When you really feel comfortable, obviously things start to flow a little bit more than usual," Stosur said.

Stosur's semi-final opponent is unseeded German Angelique Kerber, who beat Italian Flavia Pennetta 6-4, 4-6, 6-3. The pair have never played before but Stosur, seeded ninth, will start as the overwhelming favourite.

"When I came here, my goal was to get into the second or third round. Now I'm in the semi-final and it's an unbelievable feeling," Kerber said.

"I will just enjoy the match. I will play my tennis and I will try to fight."