Six months after she nearly died, Serena Williams is closing in on what could be the greatest achievement of her incredible career.

The former world number one stormed into the quarter-finals of the US Open by beating Ana Ivanovic 6-3, 6-4 on Monday and suddenly the title looks to be at her mercy.

"I'm a person that really hates to lose," Williams said.

"It's really important for me just to look at the mountain and keep climbing it, not get discouraged."

Williams's ability to overcome adversity has helped make her the finest player of her generation, and among the best of all time, but when doctors found a life-threatening blood clot on her lungs in March, winning the US Open was the last thing on the American's mind.

Yet here she is, riding high on emotion and with momentum building behind her.

Ivanovic, the 2008 French Open champion, loomed as a dangerous opponent, but Williams disposed of her with the ruthless determination of a woman on a mission.

"I think I'd like to say it's a bigger mountain like Everest. It's something you can always reach (for) and achieve whether I'm playing tennis or doing something else," Williams said.

"I don't ever want to get to the top of the mountain. I want to keep being able to reach something."

Williams went into the match as favourite, having won her two previous matches with Ivanovic, but Mother Nature was against her this time.

The wind was blowing hard at Flushing Meadows, making serving difficult. The gusts should have reduced the effectiveness of one of Williams's biggest weapons, but the 29-year-old handled the conditions better than her opponent.

Williams served nine aces and one double fault while Ivanovic landed just three aces and made eight double faults.

That was enough to give Williams the advantage but when Ivanovic made more than twice as many unforced errors as the Americans, the match all but ended as a contest.

"As I said even before the match, I think she's the favourite for the tournament," Ivanovic said.

"She's been playing really well, and it's gonna be tough (for anyone to beat her)."

Williams's next opponent is Russia's Anastasia Pavlyuchenkova, who avenged her agonising loss to Italy's Francesca Schiavone at this year's French Open to win 5-7, 6-3, 6-4.

It was sweet revenge for the Russian after she blew her chance of a first grand slam semi-finals appearance when she lost to Schiavone in the quarters at Roland Garros after leading 6-1 4-1.

"Of course I was thinking about it," Pavlyuchenkova said. "But it made me stronger I think. I never try to find excuses when I lose so I always blame myself."

Novak Djokovic is also edging towards the men's title. The Serbian has been almost invincible this year and is through to the last eight without dropping a set, although he had to survive an epic tiebreaker to maintain his perfect record.

The World No 1 survived a 30-point classic against Ukraine's Alexandr Dolgopolov, winning 16-14, and pumped his first like he had won the final. He ran away with the next two sets to win 7-6, 6-4, 6-2.

"This is one of the longest tie-breaks I ever played. It was certainly exciting to be part of it but I knew that I needed to win that set," Djokovic said.

"After I won first set, I felt that I had a little mental advantage over him, that I'm in control."

Djokovic's next opponent is his Davis Cup teammate Janko Tipsarevic, who beat former world number one Juan Carlos Ferrero of Spain 7-5, 6-7, 7-5, 6-2 in a slugfest that last almost four hours.

"It means there's going to be at least one Serbian in the semi-finals, which is great for our country," Djokovic said.

"We are great friends. It's not going to be easy to play him, but look, it's the quarter-finals and we both want to win."

The floodlit evening session was delayed by more than an hour and a half after 11th-seeded Frenchman Jo-Wilfried Tsonga needed almost four hours to beat American eighth seed Mardy Fish 6-4, 6-7, 3-6, 6-4, 6-2 in an enthralling center court clash.

Tsonga's reward for winning is a quarter-finals clash with either Roger Federer or Juan Monaco of Argentina on Wednesday.

World No 1 Caroline Wozniacki was two games away from being dismissed by former champion Svetlana Kuznetsova in straight sets before roaring back to reach the quarter-finals of the US Open on Monday.

The 15th-seeded Kuznetsova, the 2004 winner, won six of the last seven points of their first-set tiebreaker and led 4-1 in the second set before the Danish top seed recovered for a 6-7 7-5 6-1 win under the lights at Arthur Ashe Stadium.

Wozniacki broke the 26-year-old Russian's serve four times in a row to charge back in the second set and claim a 2-0 lead in the third on her way to a victory that kept alive her hopes of claiming a first major title.

The victory put 21-year-old Wozniacki, winner of six tournaments this year, into the last eight against Germany's Andrea Petkovic, a 6-1, 6-4 winner against Carla Suarez Navarro of Spain in her fourth-round match.