INDIAN WELLS, CA, USA -- No.20 seed Garbiñe Muguruza of Spain moved into the fourth round of the BNP Paribas Open on Sunday, advancing past fellow former World No.1 player and No.10 seed Serena Williams of the United States, after the American retired from the match while trailing 6-3, 1-0, due to a viral illness.

Two-time Indian Wells champion Williams led 3-0 before Muguruza reeled off six straight games to take the first set, as 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 at Indian Wells for the fourth time in the Spaniard's career.

"I think she started playing well, right away with her serve, with her shots, being very dominating," Muguruza told the press, 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."

As it happened: Muguruza advances to Round of 16 after Serena forced to retire in Indian Wells

Muguruza and Williams had met five times previously -- all at Grand Slam events, including the 2015 Wimbledon final, won by Williams, and the 2016 French Open final, won by Muguruza. On Sunday in Indian Wells, Muguruza broke the American superstar three times in succession, claiming her first victory over a Top 10 player in over a year and leveling their head-to-head at 3-3.

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

"I think I played well one set. 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."

Williams was facing her second straight former World No.1 of the event following her classic second-round win over Victoria Azarenka, but she did not reach full strength on her serve against Muguruza, winning only two of her 12 second-serve points.

"Before the match, I did not feel great, and then it just got worse with every second; extreme dizziness and extreme fatigue," Williams stated, after the match.

"By the score, it might have looked like I started well, but I was not feeling at all well physically," Williams added. "I will focus on getting better and start preparing for Miami."

Photos: From Davenport to Osaka: Indian Wells champions this century

Muguruza moves on to face No.7 seed Kiki Bertens of the Netherlands, who grinded past Great Britain's Johanna Konta on Sunday, 7-6(10), 6-4, in nearly two hours. Bertens holds a 3-1 lead in their head-to-head, but Muguruza won their most recent match in 2018.

Muguruza said that Bertens is "playing better and better," and added that she "look[s] forward to all the matches. Right now, the more the time goes by, the more that anybody can beat anybody. Doesn't matter the ranking."

Williams got off to a flying start, using crosscourt winners from both wings to reach double break point at 1-0, then breaking for 2-0 after a Muguruza forehand error. After the American held at love with a backhand winner to take a 3-0 lead, it seemed evident that the 23-time Grand Slam champion was on her way to a one-set lead.

However, Muguruza had other plans, grinding through a multiple-deuce game to get on the board at 3-1, then pulling back on serve at 3-2 after one of the Spaniard’s forehands forced an error wide from the American.

Williams began to misfire on serve, blasting two double faults in her next service game. Muguruza took advantage, converting a break point with a forehand winner to lead 4-3, finding herself up a break in the match for the first time. The Spaniard then got through one more tough service game to hold for 5-3 and put the pressure squarely on Williams.

Serving to stay in the set, the American appeared to be losing the typical zest on her storied serve, and she quickly fell behind triple set point. One long backhand miscue by Williams sealed the first set for Muguruza after a promising start by the American.

After a visit from the trainer, Williams went back on court to contest one game, but she was clearly struggling. After Muguruza held with ease to open the set, Williams elected to cease the match, putting Muguruza into the round of 16.