Roger Federer and Rafa Nadal are seemingly not the only ones sick of the sight of Novak Djokovic and his domination of men's tennis.

At the U.S. Open on Tuesday, food poisoning victim Conor Niland of Ireland threw up then gave up during his first-round match with the world number one.

Perhaps the only thing in professional tennis looking more under the weather right now is the state of the women's game. After just two days at Flushing Meadows, not one reigning grand slam singles champion is left in the draw.

Kim Clijsters, who won the U.S. Open last year and the Australian Open earlier this year, did not attempt to defend her title because of a stomach muscle problem.

On Monday, Wimbledon champion Petra Kvitova, made an inglorious early exit and was joined a day later by China's Li Na, the French Open winner.

It is little wonder that Serena Williams, less than a month away from turning 30 and in the early stages of a comeback after surviving a life-threatening lung disorder, has been installed as the favorite to win the last grand slam of the year.

In the last match of Tuesday's night session, the American sent an ominous warning to her rivals when she began her campaign with a 6-1 6-1 win over Serbia's Bojana Jovanovski.

Niland lasted only 44 minutes before he tossed in the towel against Djokovic. He was trailing 6-0 5-1 and already looking as green as the fluffy balls he was frantically trying to retrieve.

The first Irishman to play at the U.S. Open, he did at least have a valid excuse. He was suffering from food poisoning after eating a meal of pork and salad at a fancy Manhattan restaurant.

I thought I could bluff my way through but you can't do that against the number one in the world, I just found out, Niland said.

It was not a fair contest anyway. Djokovic, his appetite for more grand slam titles still driving him, was in a ruthless mood and showing little charity to his ailing opponent.

He showed no ill effects of the shoulder injury that forced him to quit last week's Cincinnati final against Andy Murray but was glad to get an early mark on a hot and sunny afternoon at Flushing Meadows.

STRAIGHT SETS

I really don't mind that I spend less time on the court, Djokovic said.

The Serbian has lost just two matches this year, claiming the Wimbledon and Australian Open titles along the way, but his intentions for the next two weeks are clear and his main rivals have been put on notice.

Now, more than ever, I know that I can actually perform equally well on any surface, he said.

Nadal began his title defense with a straight sets win over Andrey Golubev of Kazakhstan. The Spaniard won 6-3 7-6 7-5 but it was a performance that raised as many questions as answers.

Last year, Nadal completed his personal grand slam after working tirelessly on his serve, turning a shot that had been a weakness into a weapon. His serve was broken just five times in the seven matches he played last year.

Against Golubev, ranked 98th in the world, his serve was broken six times.

It's a positive start, Nadal said. It was unbelievable that I won straight sets, but it is still a victory in straight sets.

Caroline Wozniacki, the women's world number one, provided a glimpse of the ruthless streak her critics have accused her of lacking, demolishing unseeded Spaniard Nuria Llagostera Vives 6-3 6-1.

The Dane, who has been in the headlines as much for her relationship with golfer Rory McIlroy as her performances on court, smashed 22 winners in a lopsided victory that took just 80 minutes.

It was as impressive a start to the tournament as anyone has made so far but Wozniacki was forced to defend herself, and by extension the women's game, because she has still not won a grand slam title.

They can say what they want, she snarled. I've won a lot of tournaments. I'm number one in the world.

Li is one of the most marketable players in the game since becoming the first Chinese to win a grand slam singles crown but has struggled since, and joined the list of big casualties at the tournament when she fell 6-2 7-5 to unseeded Romanian Simona Halep.

Williams left the U.S. Open in disgrace two years ago after a foul mouthed tirade on an official that earned her a hefty fine and a two-year probation, which ends after this tournament.

But the American was on her best behavior against Jovanovski, racing to victory in less than an hour then sweet-talking the Arthur Ashe Stadium crowd.

I'm so happy to be here, I didn't think I would make it, she said in an on-court interview.