Roger Federer and Serena Williams both swept into the third round of the US Open on Thursday with the sort of ruthless performances that have made them the king and queen of tennis.

The former world number ones showed no mercy against their opponents as they sent an ominous warning to rivals of their grand intentions for the last major championship of the year.

Federer, already a five-time champion at Flushing Meadows, demolished Israel's Dudi Sela 6-3, 6-2, 6-2, in just 77 minutes on the Arthur Ashe Stadium. The Swiss master was untroubled and offered a blunt assessment of the contest.

"When it's like that, obviously it's tough for the opponent," Federer said. "But I just think I was superior today."

Williams, also on center court, was just as ruthless. She thrashed Michaella Krajicek of the Netherlands 6-0, 6-1 in less than an hour, racing around like the court like a teenager instead of a 29-year-old on the comeback trail.

"One way to get faster is you can pretend there's a check for a million dollars there tax-free, and just go run it down," she explained.

But Federer, 30, and Williams, who turns 30 later this month, are now being hunted by younger rivals.

Novak Djokovic, who is the world number one and the favourite, got a full match in but barely a full workout as the world number one romped past unseeded Carlos Berlocq of Argentina 6-0, 6-0, 6-2 on Thursday to advance to the third round.

The Serb had won his first round match when Irish qualifier Conor Niland retired trailing 6-0, 5-1 over worries he would get sick on court from a bad case of food poisoning.

Australian Open and Wimbledon champion Djokovic spent a bit more time on Arthur Ashe Stadium court against Berlocq, with the greatest suspense being whether the soft-hitting Argentine would be able to win a game from the top seed.

Berlocq broke through in the 15th game and won a standing ovation from the centre court crowd.

Djokovic next plays Russian Nikolay Davydenko, who advanced past Potito Starace of Italy in straight sets.

Women's world number one, Caroline Wozniacki, was also impressive in her second round match against Arantxa Rus of the Netherlands.

After a slow start in which she dropped early service games, she ran away with the match 6-2, 6-0 in 63 minutes.

"I feel like I'm on a roll and I'm playing well," Wozniacki said.

"I believe I can beat anyone on a good day. But I have to play on a high level for seven matches, and that's not easy."

There were no major surprises on Thursday although four lower seeded players were knocked out.

Michael Llodra of France was beaten and Czech Radek Stepanek retired with an injury, while on the women's side Estonia's Kaia Kanepi and Shahar Peer both fell.

Mardy Fish, one of the few men given any hope of challenging Federer, Djokovic, Rafa Nadal and Andy Murray, sailed through with a clinical 6-2, 6-2, 6-4 win over Tunisia's Malek Jaziri.

"I haven't really been tested that much," said the eighth-seeded American. "I can't do anything about that. I can't do anything about who I come up against. Just happy to move on."

Victoria Azarenka, the fourth seed from Belarus, also impressed with her 6-4, 6-3 win over Argentina's Gisela Dulko and now faces Serena Williams, seeded 28th, in the third round.

Williams spent almost as long in her news conference talking about her sister Venus's health problems than she did on court, but said she had not allowed herself to be distracted by it.

"It really wasn't that difficult, to be honest. I mean, she wants me to do the best, she wouldn't want me to suffer," Serena said.

"If anything, it should motivate me more."

Venus pulled out of the Open on Wednesday after revealing she was suffering from Sjogren's Syndrome, an autoimmune disease that causes fatigue and joint pain.