American teenager Christina McHale stunned Wimbledon champion Petra Kvitova with a 2-6, 6-2, 6-3 victory in the night session of the PNB Paribas Open, which has seen several withdrawals because of illness.
Gael Monfils of France, Jurgen Melzer of Austria and American Vania King each battled an intestinal virus that has sidelined at least a half-dozen players.
Melzer did play his singles match but lost in straight sets before withdrawing from doubles.
Vera Zvonareva also withdrew because of a virus, but reportedly her ailment was not the same one that afflicted the other players.
Kvitova has been trying to overcome a virus she contracted while in the Middle East, and that manifested itself when she felt a loss of energy and movement after a good first set against the 19-year-old McHale
''I lost a little bit of my fitness so it was tough moving and having winners,'' she said. ''(McHale) was moving very well so she played a lot of shots back. (And) my serve was not really good, too. She played very solid and she didn't make mistakes so it was tough.''
McHale will face No. 18 Angelique Kerber of Germany in the fourth round.
''I couldn't get a read on (Kvitova's) serve in the first set and I wasn't really moving very well, either,'' McHale said. ''Then I just kind of hung in there early in the second. She played one service game where she gave me a couple of loose errors and then I got more comfortable as the match went along.''
The top seeds have remained healthy, although 16-time Grand Slam champion Roger Federer admitted to not feeling well after his 6-4, 6-1 victory over American wild card Denis Kudla in the second round.
The No. 3-ranked Swiss appeared pale and listless when he walked into the interview room, and there were reports he had suffered some stomach problems on court.
Federer, a three-time Indian Wells champion, said he did have a fever.
''It's not a terrible temperature, but there's something going on,'' Federer said, attributing the illness to contact with his family and not a result of the intestinal virus.
Rafael Nadal, playing singles for the first time in six weeks, also advanced to the third round with a 6-1, 6-3 victory over Argentina's Leonardo Mayer.
Nadal had not played since losing to top-ranked Novak Djokovic in a nearly 6-hour marathon final at the Australian Open on Jan. 29. The Spaniard was in control throughout the match with Mayer, who fell to 0-8 in his career against top 10 opponents.
''It's nice to be back playing at the very good level, in my opinion,'' Nadal said after not facing a break point in the 75-minute match. ''Winning is the most important thing. It's not easy to be back after a while without competition but I did, I believe, really well.
''I started the match very focused, very aggressive. Today the only thing I can say, I'm very happy about almost everything.''
Nadal will play countryman Marcel Granollers in the third round and has a potential semifinal date with Federer.
David Ferrer of Spain, who could face Federer in the quarterfinals, was a 6-2, 6-2 winner over Grigor Dimitrov of Bulgaria, and Jo-Wilfried Tsonga of France got a quick victory over countryman Michael Llorda, who retired with a knee injury after falling behind 4-1 in the opening set.
In other action, Argentina's Juan Martin del Potro beat Australian qualifier Marino Matosevic 7-5, 6-2, and No. 10 Janko Tipsarevic of Serbia was a 6-4, 6-2 winner over Gilles Muller of Luxembourg.
On the women's side, top-ranked Victoria Azarenka of Belarus got little resistance from Svetlana Kuznetsova, beating the Russian 6-1, 6-2.
Li Na of China beat countrywoman Jie Zheng 6-1, 6-3, Julia Goerges of Germany was a 6-3, 6-4 winner over Anabel Medina Garrigues of Spain, and Kerber advanced on a walkover when King withdrew.