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.

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 at Centre Court, too.