Marta Kostyuk had just one day to prepare for her match on Sunday at the WTA Finals, and there were moments when it showed in a 6-6, 6-4 loss to eighth-seeded Linda Noskova at the Indian Wells Tennis Garden.

Kostyuk replaced top-seeded Elena Rybakina, last year’s defending champion and the top-ranked player in the world. Rybakina withdrew Saturday morning in what was described by the WTA as a “viral illness.” Kostyuk was already on-site as the first alternate.

The Ukrainian tennis star, ranked 10th in the world, was slow to start Sunday’s match. She initially struggled with her serve, immediately falling behind a break. She got back on serve the following game but then went down a break again in the third game.

Kostyuk got back on track in the second set, but Noskova broke to take a 4-2 lead and the Czechia tennis star held serve the rest of the way.

The match was the first to kick-off a round robin format at the WTA’s end-of-the-year tournament. The event is being held in Indian Wells this year, after being held the two previous years in Riyadh.

The tournament includes the top-eight ranked women in the world and the top-ranked doubles teams and continues until Nov. 15.