WASHINGTON, March 10 (Xinhua) -- No.20 seed Garbine Muguruza of Spain moved into the fourth round of the BNP Paribas Open on Sunday, advancing past former world number one Serena Williams after the American retired from the third round match while trailing 6-3, 1-0 citing a viral illness.

Two-time BNP Paribas Open champion Williams led 3-0 before Muguruza reeled off six straight games to take the first set. The American increasingly struggled with her illness as the match progressed. Following one game in the second set, Williams was unable to continue, putting Muguruza into the fourth round.

"I think she started playing well, right away with her serve, with her shots, being very dominating," Muguruza said after the match. "I had to adapt a little bit my position in the court, the way I was hitting, and it took me a few games to kind of do it. Once I did it, I felt much more comfortable to be able to release my tennis."

"It's really a weird feeling, because I don't feel like I won the match point and, 'Well done, good match,'" said Muguruza. "It was just like, 'Man, we'll play next time.'

"I'm going to take the positive side of that. Now I have more chances to face top opponents. That's what I'm looking for," Muguruza added.

Williams said: "Before the match, I did not feel great, and then it just got worse with every second. Extreme dizziness and extreme fatigue. By the score, it might have looked like I started well, but I was not feeling at all well physically."

"I will focus on getting better and start preparing for Miami," she added.