Karolina Muchova withdrew from the WTA Finals on Friday due to a gastrointestinal illness, the Associated Press reported.
The 30-year-old Czech player was scheduled to compete against Mirra Andreeva during the final day of round-robin action at the Indian Wells Tennis Garden.
Because Muchova had already lost her first two matches, she was mathematically unable to advance to the semifinals regardless of the outcome, according to the news report.
Diana Shnaider, the second alternate, stepped in to make her singles debut at the tournament, the agency noted.
Shnaider previously qualified for the event in doubles alongside Andreeva, though the duo went winless during their previous appearance.
The match provided significant financial incentive for Shnaider, who stood to earn 200 ranking points and $230,000 for a victory, in addition to a $220,000 participant fee.
This tournament has faced multiple roster challenges, as Muchova is the second player to withdraw from the $10 million event, as documented by the Associated Press.
Top-ranked Elena Rybakina dropped out before the competition began last Sunday, citing exhaustion and a viral illness, per reporting from the AP.
Marta Kostyuk replaced Rybakina in the draw, finishing with a 0-3 record in Group 1 play.
This marks the first time since 2019 that both singles alternates have been required to compete during the round-robin stage.
Meanwhile, the tournament continues as Linda Noskova and Jessica Pegula have advanced to the semifinals from Group 1.
Andreeva secured the top spot in Group 2 with a 2-0 record, while the final semifinal berth remains to be determined by a match between Aryna Sabalenka and Elina Svitolina.