STANFORD -- Even as she wobbled on the court in a frightening display that sparked worldwide attention and concern, Serena Williams kept trying to play.
"That's the fight in me," Williams said Monday, publicly addressing her bizarre exit from Wimbledon for the first time. "I am always go, go, go. I never stop. I never want to give up. I never know when to say when."
Suffering from what officials later called a viral illness, Williams spent the next three days in bed as many, including some of the sport's legendary names, speculated whether the world's No. 1-ranked player was dealing with something other than a virus.
Williams, 32, seemed recharged Monday as she prepares for her opening match Wednesday night at the Bank of the West Classic, her first since withdrawing from the now infamous Wimbledon doubles match July 1. The Bank of the West's top seed did not give a specific diagnosis but said she would undergo more medical tests after the tennis season.
She vacationed in Europe after Wimbledon, mixing in sun and fun with early-morning practices.
"I am feeling pretty good about my game," Williams said. "But as you know, I am never feeling great about it."
Nobody felt great about Williams' game when she struggled on the Wimbledon grounds to do the most basic tennis tasks, such as tossing the ball for her serve.
Playing alongside her older sister, Venus, who will play at the Bank of the West on Tuesday, Williams lasted three games before officials stopped the match.
Venus earlier told her sister to walk off the court. "I think she almost punched me," Williams said.
But Williams continued because, in doubles, she needed to cover only half the court.
It was not until later, Williams said, that she realized the seriousness of the situation.
"I was really, really sick," Williams said. "Literally the next three days I couldn't get out of bed. Usually when you lose in a tournament, you just leave. I literally stayed until the tournament ended because I was not allowed to leave. Doctors told me you've got to stay."
In 2011, the 17-time Grand Slam singles champion had blood clots in both lungs and a hematoma in her abdomen.
"It kind of reminded of when I had my other illness a while ago," Williams said. "I didn't realize how sick I was until my dad came over and we were walking outside for no more than three minutes and I said, 'OK, I've got to go inside.' I slept the whole day.
"After this happened, I kind of had that flashback where you're in this moment and you don't realize how sick you are. You kind of step back and look at everything and picture everything."
The Wimbledon incident prompted widespread speculation.
"I find it distressing," Martina Navratilova told ESPNW later that week. "I think virus, whatever they're saying it was, I don't think that was it. I think it's clear that's not the case. I don't know what it is, but I hope Serena will be OK. And most of all, I don't know how she ended up walking onto the court."
Williams said she heard about the speculation weeks later.
Her response: "Oh really?"
Angelique Kerber, the Bank of the West's third seed, watched Williams practice Monday and walked away impressed.
"She was back, for sure," Kerber said.
Williams won the tournament at Stanford in 2011 and 2012.
"I feel really good about this event and this tournament and being here," Williams said. "It's always been a special place for me."
The Associated Press contributed to this report. Follow Darren Sabedra on Twitter at twitter.com/DarrenSabedra.
Monday's top results
Naomi Osaka d. Samantha Stosur 4-6, 7-6 (7), 7-5
No. 8 Andrea Petkovic d. Ajla Tomljanovic 6-1, 6-4
Coco Vandeweghe d. Kristie Ahn, 6-2, 7-5
Tuesday's key matches
No. 6 Dominika Cibulkova vs. Garbine Muguruza
No. 5 Ana Ivanovic vs. Sabine Lisicki
Venus Williams vs. Paula Kania
Daniela Hantuchova vs. Paula Ormaechea