Douglas Robson

USA TODAY Sports

STANFORD, Calif. – In her first public comments since her troubling exit from Wimbledon earlier this month, No. 1 Serena Williams said she plans to undergo further medical testing and would not compete under similar conditions again.

"Now I know better if I'm feeling bad I'm not going to step on the court," Williams said Monday at the Bank of the West Classic, where she is a two-time champion (2011-12) and riding a nine-match winning streak after skipping last year's tournament.

Interest has been intense since Williams, appearing clumsy and disoriented, retired from a second-round doubles match at Wimbledon with sister Venus Williams. Two days earlier the 17-time Grand Slam champion lost in the third round of singles to France's Alize Cornet.

Speaking with reporters and also separately to USA Today Sports, Williams said the viral illness that felled her in London had left her bedridden for three days and under doctors' orders not to travel.

"I was really, really sick," she said. "Especially me," she added, "when I lose I'm on the very next flight out of there."

Williams, whose sister Venus suffers from the energy-sapping autoimmune disease Sjogren's syndrome, said she would have more testing after the season "for things that do run in the family" but couldn't provide a more specific diagnosis.

She said she had continued to play at Wimbledon despite sometimes struggling to make contact with the ball because she is a fighter who doesn't like to disappoint.

"I never want to let people down," she said. "I never want to let my sister down. I don't want to let the fans down. I was thinking I don't want anyone to be disappointed after I lost in singles."

Williams, 32, said she hadn't fully recognized how ill she was and compared it to 2011 when she was hospitalized with blood clots and a hematoma that had to be surgically removed.

"I was really scared after because I didn't realize how I felt until later," she said.

Williams said she does not read the press and hadn't focused on speculation from various corners about what had caused her strange behavior.

But offering a few more details, she said she had started to feel poorly the day after losing to Cornet and didn't bother to warm up before her doubles match. She said she had taken no medication before taking the court, where she appeared to cry and was treated by a trainer who took her blood pressure prior to play.

Later, her coach of two years, Patrick Mouratoglou, said he had not seen Williams in 48 hours. Mouratoglou eventually walked a shaky looking Williams to a waiting car that whisked her off the grounds.

Asked if she was in both physical and emotional distress, Williams said: "I would say it was definitely more physical because after my match against Cornet I wasn't feeling great and it just got worse. I didn't get out of bed even before the match. I was feeling awful. Then after the match I wasn't eating, I wasn't drinking. I wasn't anything. I was super dizzy."

After pulling out of the event at Bastad, Sweden, following Wimbledon, Williams took an impromptu holiday to Croatia on the recommendation of her Serbian-born hitting partner, Sascha Bajin.

"I went on what I call a vacation," she said using air quotes, "which is like you have fun in the sun but you practice in the day in the morning."

She also flew to Toronto, where she filmed some scenes for an upcoming but undisclosed movie in which she plays herself.

"I think I was definitely really rusty," said Williams, who has dabbled in acting.

Williams, looking fit and cracking the occasional joke, said she was hungry and eager to get back to business after failing to advance past the fourth round in three majors so far this year.

The two-time champion opens Wednesday night in the second round against No. 45 Karolina Pliskova of the Czech Republic, who beat Kimiko Date-Krumm of Japan 6-1, 6-3 in the first round Monday.

"I feel really good," Williams said. "I feel really happy to be here."

"She's back for sure," said No. 3 seed Angelique Kerber of Germany. "I watched her practice this morning."