Defending champion Elena Rybakina withdrew hours before her opening match at the ATP-WTA Indian Wells Masters on Friday with a gastrointestinal illness.
The former Wimbledon champion from Kazakhstan, seeded fourth, had been due to open her campaign with a second-round meeting with Argentina's Nadia Podoroska.
She was replaced in the draw by American lucky loser Kayla Day.
"It is with great sadness I must announce, I will not be able to participate at this year's Indian Wells tournament due to gastrointestinal issues," Rybakina said in a statement.
"Those close to me know how much this tournament means to me and how much I wanted to come back and defend my title.
"I would like to thank all the fans who came out here to support me and I am sorry they will not get a chance to see me play this year.
"Now I will rest and recover so I can come out and play and compete for my fans to the best of my abilities once again."
Rybakina, who beat Aryna Sabalenka in last year's final, already owns two titles this season, winning at Brisbane and Abu Dhabi. But she pulled out of her scheduled quarterfinal in Dubai last month also complaining of gastrointestinal trouble.