Indian Wells: Roger Federer has so far survived the odds at Indian Wells, having battled flu and a temperature to beat three opponents and reach the quarter-finals at the elite ATP tournament. (Agencies)
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."
Federer, whose entire family has been battling flu symptoms, eased past American wild card Denis Kudla 6-4, 6-1 in his opening match at the Indian Wells Tennis Garden.
He then survived an early storm from Canadian Milos Raonic before completing a 6-7, 6-2, 6-4 victory on Tuesday to advance into the fourth round.
On Wednesday, he initially struggled against 50th-ranked Brazilian Bellucci before breaking his left-handed opponent in the first and ninth games of the second set to level the match.
The final set went with serve until Federer again broke the South American in the 10th game, converting his second match point when Bellucci hit a forehand long.
"At the end I found a way, dug deep and came through," said the 30-year-old Federer, who won three consecutive titles at Indian Wells from 2004-06. "It's really important.
"These are the wins that almost feel better, to be quite honest, because when you're playing great, it's simple, it's easy, right? But when it's not going your way and things are difficult, that's kind of the good wins."
Federer, who has won 36 of his 38 matches since last year's U.S. Open, will face 2009 U.S. Open champion Juan Martin del Potro of Argentina in the last eight.
Indian Wells: Roger Federer has so far survived the odds at Indian Wells, having battled flu and a temperature to beat three opponents and reach the quarter-finals at the elite ATP tournament.