WIMBLEDON, England >> Serena Williams bit her upper lip. She held her left hand over her mouth and tried to hold back tears while getting ready to serve.

It was the first set of her first-round match today at Wimbledon, and Williams knew this stay at a tournament where she has won seven of her 23 Grand Slam singles titles was about to end because she hurt her right leg when she lost her footing behind a baseline.

Moments later, her legs buckled as she tried to change directions to chase a shot by her opponent, 100th-ranked Aliaksandra Sasnovich of Belarus. Williams dropped to her knees, her head down on the grass. She used her racket to help her stand, but only so she could limp to the net to concede — just the second mid-match retirement at any Grand Slam tournament of her career and first since 1998.

“I was heartbroken to have to withdraw today,” Williams said in a statement released by the tournament.

“Feeling the extraordinary warmth and support of the crowd today when I walked on — and off — the court,” she said, “meant the world to me.”

Said Sasnovich: “She’s a great champion, and it’s (a) sad story.”

Roger Federer surely articulated a common sentiment when told by a reporter what happened to Williams.

“Oh, my God,” he said. “I can’t believe it.”

Williams was serving while leading 3-1 at Centre Court — where the retractable roof was shut because of rain that forced the postponement of two dozen matches until Wednesday — when her left shoe seemed to lose its traction while she was hitting a forehand.

Williams winced and stepped gingerly between points, clearly troubled. After dropping that game, she asked to visit with a trainer and took a medical timeout.

She tried to continue playing. The crowd tried to offer support and encouragement. Eventually, the 39-year-old American couldn’t continue. The chair umpire climbed down to check on her, and they walked together up to the net; the score was 3-all, 15-30 when Williams stopped.

Williams, who began the match with her right thigh heavily taped, raised her racket with right arm and put her left palm on her chest. Then she waved to the spectators.

Officially, this goes in the books as only the second first-round Grand Slam exit of Williams’ career. The other came at the 2012 French Open, where she was beaten by Virginie Razzano. Shortly after that, Williams teamed up with coach Patrick Mouratoglou and began accumulating majors to eclipse Steffi Graf’s professional era record of 22 and move within one of Margaret Court’s all-era mark of 24.

“All the best for her,” said Sasnovich, who reached the fourth round at Wimbledon in 2018 for her best Grand Slam result.

Williams’ departure makes a wide-open women’s draw even more so. As it was, defending champion Simona Halep and four-time major champ Naomi Osaka withdrew before the tournament started.

And so, even as her 40th birthday approaches in September, Williams was among the top contenders. With her best-in-the-game serve and stinging groundstrokes, she had made it to the past four finals when she entered Wimbledon — winning in 2015 and 2016, missing the tournament while pregnant in 2017, then finishing as the runner-up in 2018 and 2019 (it was canceled last year because of the pandemic).

Williams was hardly the first player to find it difficult to deal with the slick grass over the first two days of main-draw play.

In the match that preceded hers in the main stadium, eight-time Wimbledon champion Federer advanced when his opponent, Adrian Mannarino, injured his right knee late in the fourth set when he tumbled near the same spot Williams did.

Federer was trailing two sets to one, but ahead 4-2 in the fourth, when Mannarino fell. He tried to continue but dropped eight of nine points when they resumed and called it quits.

“Obviously,” Federer acknowledged, “he was the better player.”

Novak Djokovic fell twice in the first set of his first-round victory Monday at Centre Court, too.

“I do feel it feels a tad more slippery, maybe, under the roof. I don’t know if it’s just a gut feeling. You do have to move very, very carefully out there. If you push too hard in the wrong moments, you do go down,” Federer said. “I do feel it’s drier during the day. With the wind and all that stuff, it takes the moist out of the grass. But this is obviously terrible.”

It was, by far, the most significant development today, when the winners included Williams’ older sister, 41-year-old Venus, 17-year-old Coco Gauff, reigning French Open champion Barbora Krejcikova and No. 1 seed Ash Barty in the women’s bracket, and No. 2 Daniil Medvedev, No. 4 Alexander Zverev and No. 10 Denis Shapovalov in the men’s.

Sebastian Korda — a 20-year-old American whose father, Petr, won the 1998 Australian Open and whose sisters, No. 1-ranked Nelly and No. 13 Jessica, are on the LPGA Tour — made a successful Wimbledon debut, eliminating No. 15 seed Alex de Minaur 6-3, 6-4, 6-7 (5), 7-6 (5).

Venus Williams accumulated 10 aces by smacking serves at up to 114 mph — not quite like the old days, but not too shabby, either. She drove forehands to corners. She made her way to the net for crisp volleys.

And when it was all over, she celebrated her first Wimbledon match win since 2018 by raising her arms and yelling “Come on!” before reprising her familiar smile-and-twirl wave at No. 3 Court.

A five-time singles champion at the All England Club who is making her 23rd appearance here, the elder Williams sister began her record-extending 90th Grand Slam tournament with her 90th career victory at Wimbledon, beating Mihaela Buzarnescu of Romania 7-5, 4-6, 6-3.

Venus Williams is a former No. 1-ranked player who came into this week ranked 111th and having lost in the first or second round at the past eight majors. That included a first-round exit in 2019 at the All England Club against a then-15-year-old Gauff.

“You can’t win them all. Life is about how you handle challenges. Each point is a challenge on the court. No one gives you anything,” said Venus Williams, who was diagnosed a decade ago with Sjogren’s syndrome, an autoimmune disease that can cause fatigue and joint pain. “I like to think I handle my challenges well.”