(CNN) -- A mystery stomach bug is striking down men's and women's players at the Indian Wells Masters tennis event, but former world No. 1 Roger Federer shook off illness to cruise to victory in his opening match.
Federer, a three-time winner of the U.S. hard-court tournament, has been suffering with flu-like symptoms but breezed past Ukraine-born American wildcard entrant Denis Kudla on Sunday.
The third-ranked Swiss won 6-4 6-1 in 57 minutes to set up a third-round clash with Canadian rising star Milos Raonic.
The 16-time grand slam winner was one several players to fall sick at the California event, but Federer managed to avoid the intestinal illness which forced some of them to withdraw on Sunday.
France's world No. 14 Gael Monfils and Austrian Jurgen Melzer were lost from the men's draw, while ninth-ranked Russian Vera Zvonareva and American Vania King pulled out of the women's tournament.
Philipp Kohlschreiber, Andreas Seppi and Magdalena Rybarikova had earlier withdrawn.
"We're fighting something of our own in our family," Federer, who has twin girls, told reporters. "We have tons of stuff going on at the moment.
"I have a bit of a combination. Not terrible temperature, but I have some things going on. But I'm the best off of the family, so thank God I'm the tennis player here. No, the rest of them are struggling much more."
Raonic, winner of two ATP Tour titles this season and seeded 27th, advanced courtesy of a 6-4 6-2 win over Argentina's world No. 38 Carlos Berlocq.
World No. 2 Rafael Nadal was in similarly impressive form, brushing aside 78th-ranked Argentine Leonardo Mayer 6-1 6-3 in his first match since losing an epic Australian Open final to world No. 1 Novak Djokovic.
Nadal is also participating in the doubles event and won his first-round match alongside compatriot Marc Lopez last week.
"It's nice to be back playing at a very good level," said Nadal, a three-time winner of this event who lost in last year's final to Djokovic.
"Winning is the most important thing. It's not easy to be back after a while without competition, but I did really well. I started the match very focused, very aggressive. I'm very happy about almost everything."
Next up for the 10-time grand slam champion is 26th-seeded compatriot Marcel Granollers, who beat Germany's former world No. 2 Tommy Haas 6-3 2-6 6-3.
On a day of withdrawals, French sixth seed Jo-Wilfried Tsonga advanced after compatriot Michael Llodra was forced to retire with a knee injury while trailing 4-1 in the first set of their match.
World No. 41 Llodra has reportedly been slapped with a $2,500 fine for racially abusing a member of the crowd during his victory over Latvia's Ernests Gulbis on Friday.
Tsonga's next opponent will be Czech 28th seed Radek Stepanek, who beat Belgium's Xavier Malisse 6-2 6-1.
Argentine ninth seed Juan Martin del Potro beat Australian qualifier Marinko Matosevic 7-5 6-2 to continue his bid for a third title of 2012.
The 2009 U.S. Open champion will next play Spanish No. 19 Fernando Verdasco, who beat American Ryan Sweeting 6-2 6-2.
In the women's draw, 2011 Wimbledon champion Petra Kvitova was shocked by American teenager Christina McHale, slipping to a 2-6 6-2 6-3 defeat in their third-round clash.
"She played very solid and didn't make any mistakes, so it was tough," world No. 3 Kvitova said of the 32nd seed.
"The first set was fine today. Next time I play against her I will have to play the game I played in the first set -- a better serve, not many double faults, play aggressive and go for the net."
McHale, 19, who is searching for her first WTA title, said: "It took me a while to get used to her big shots, but I definitely got a lot more comfortable as the match went on. It was definitely one of my biggest wins."
McHale will play 18th seed Angelique Kerber in the fourth round, after the German benefited from the withdrawal of King.
World No. 1 Victoria Azarenka is safely through after a 6-1 6-2 victory over Russian 25th seed Svetlana Kuznetsova. The Australian Open champion's next opponent will be 14th seed Julia Gorges, who beat Spain's No. 24 Anabel Medina Garrigues.
Chinese eighth seed Li Na beat compatriot Zheng Jie 6-1 6-3, and the French Open champion will next play Czech Klara Zakopalova, who profited from Zvonareva's withdrawal.
Polish fifth seed Agnieszka Radwanska set up a clash with American wildcard Jamie Hampton by beating Italian 27th seed Flavia Pennetta 6-4 6-2.