Carlos Alcaraz is struggling with stomach problems after losing to Casper Ruud in the opening game of the ATP Finals

TURIN, Italy (AP) — Carlos Alcaraz, battling a stomach problem, appeared far from the player who won two Grand Slams this year in his 6-1, 7-5 loss to Casper Ruud at the ATP Finals.

It was Ruud’s first win over Alcaraz in five meetings in his career and the result could make it difficult for Alcaraz to progress beyond the round-robin stage of the season-ending top eight player tournament.

“I didn’t feel comfortable on the pitch,” said Alcaraz. “A few days before I came here, I got sick at home. … This morning I feel uneasy in my stomach.”

Alcaraz won the French Open and Wimbledon, increasing his career total to four Grand Slam titles. But he also recently had problems at the Paris Masters and fell behind Alexander Zverev to third place in the rankings this week.

“I have no experience playing on indoor courts,” Alcaraz said. “I’ll be a really good player on indoor courts, I’m sure of that. But I think it’s time to gain experience. … Many players are better than me on the indoor court.”

Ruud reached the 2021 semifinals and the 2022 final in his first two finals appearances. But he too has struggled recently, losing his first or second game in his last seven tournaments.

“I’m not full of confidence,” Ruud said. “A game like today could change that.”

Alcaraz was broken as he served the second set 5-4.

Alcaraz led 25-16 but committed 26 unforced errors while Ruud made 10 errors.

Ruud converted all four of his break points, while Alcaraz was only one of six in this category.

“I’m mentally tired,” Alcaraz said. “Obviously a lot of games, a really tight schedule, a really challenging year with not too many days off.”

Later in the group of Alcaraz and Ruud, Zverev plays Andrey Rublev.

The top two finishers in each group advance to the semi-finals.

In the other group, leader Jannik Sinner and US Open finalist Taylor Fritz beat Alex de Minaur and Daniil Medvedev, respectively.