Roger Federer of Switzerland plays a backhand shot during his win over Tomaz Bellucci of Brazil at the BNP Paribas Open in Indian Wells, California, on Wednesday.

Roger Federer has so far survived the odds at Indian Wells, having battled flu and a temperature to beat three opponents and reach the quarterfinals at the BNP Paribas Open in Indian Wells.

A triple champion at the desert venue, Federer had initially been worried he would not be fit enough to compete in his opening match but he managed to get through that and has since felt increasingly better.

"I'm still hurting a bit, but at least I don't have a massive headache, aches and pains anymore," the third-seeded Swiss told reporters after surviving a shaky first set to beat Brazilian Thomaz Bellucci 3-6, 6-3, 6-4 on Wednesday.

"That's all gone now. I got maybe a bit lucky but it's taken some time to for me to get better, to be honest. I thought I would see quicker improvements."

Asked if he had considered withdrawing from the ATP Masters 1000 event, Federer replied: "The night was rough before my first-round match, and I was thinking, 'I don't know how I'm gonna do this.' But I woke up and I felt a little bit better.

"That went and I didn't feel good in the afternoon again. One hour before the match I started to feel better again. At that point I always knew I was going to go out on court and give it a try, at least.

"I knew I had a day off after that, after the first-round match. That really gave me the belief that I could do it, you know."

Top-ranked Novak Djokovic also reached the quarterfinals, while No. 1 Victoria Azarenka improved to 21-0 on the season with a straight-set win on the women's side.

Second-ranked Rafael Nadal beat 21st-seeded Alexandr Dolgopolov of Ukraine 6-3, 6-2 to set up a quarterfinal against David Nalbandian, who upset sixth-seeded Jo-Wilfried Tsonga 3-6, 7-5, 6-3.