REUTERS: An ill Serena Williams retired from the BNP Paribas Open while trailing Spain's Garbine Muguruza 6-3 1-0 at Indian Wells on Sunday.
Tournament officials said the former world number one was suffering from a viral illness.
Williams walked to the chair after the first game of the second set of her match with Muguruza and sat down, breathing heavily. After a discussion with supervisor Donna Kelso, the American retired.
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.
She retired after losing the first game.
Williams had taken a 3-0 lead in the first set before losing the next seven games.
"I felt that maybe she wasn't feeling well," said Muguruza. "It's really a weird feeling, because I don't feel like I won the match point and (got the) 'well done, ... good match'.
"It was just like, 'man, we'll play next time'."
There was no immediate comment from Williams.
The American, who has won 23 Grand Slam singles titles, gave birth to a daughter in 2017 and returned to play last year.
She is currently ranked 10th in the world.
Williams was playing in her first tournament since the Australian Open in late January.
She had defeated Victoria Azarenka 7-5 6-3 in her first match on Friday.
(The story is refiled to amend headline to fix typo, no change to text.)
(Reporting by Gene Cherry in Raleigh, North Carolina; Editing by Ken Ferris/Greg Stutchbury)