Serena Williams has retired from her third-round match at the WTA event at Indian Wells.
Williams lost the first set 6-3 to two-time major champion Garbine Muguruza and was trailing 1-0 in the second set when she called it quits.
A trainer visited 10th seeded Williams on court before she departed. No immediate reason was given.
Williams, 37, had shown signs of fatigue during the first set and after it was completed she called for a trainer and following a lengthy conversation returned to the court but could only play one more game.
The American had taken a 3-0 lead in the first set before losing the next seven games.
Williams was playing her first tournament since the Australian Open in late January, when she lost in the quarter-finals.
She received a first-round bye in Indian Wells and then beat Victoria Azarenka 7-5 6-3 on Friday.
Spanish 20th seed Muguruza will play Kiki Bertens in the round of 16 after the Dutch seventh seed accounted for Brit Johanna Konta 7-6 (12-10) 6-4.
Earlier on Monday, world No.2 Simona Halep needed nearly two hours to beat Ukrainian qualifier Kateryna Kozlova 7-6 (7-3) 7-5 in the third round.
Kozlova led 6-5 and was within two points of winning the first set before Halep forced a tiebreaker in their first meeting.
Halep dominated the tiebreaker, urged on by a large contingent of Romanian fans at Indian Wells Tennis Garden.
“It was difficult for me to close out those sets,” Halep told reporters.
“When I was leading the match, she played without fear.”
The victory advanced the French Open champion, who won Indian Wells in 2015, to the fourth round for the fifth time in six years.
Halep’s next opponent will be the unseeded Marketa Vondrousova, who upset 22nd-seeded Jelena Ostapenko 4-6 6-3 6-4.
Meanwhile, Chinese 18th seed Qiang Wang outlasted 16th-seeded Elise Mertens 7-6 (7-4) 6-7 (4-7) 6-3.