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 Tuesday 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 injured her left 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.

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

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

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

Williams was serving while leading 3-1 at Centre Court when her left shoe seemed to lose its traction while she was hitting.