Carlos Alcaraz struggled with a stomach issue and looked far from the player who won two Grand Slams this year while losing to Casper Ruud 6-1, 7-5 at the ATP Finals.

It marked Ruud's first victory over Alcaraz in five career meetings, and the result could make it difficult for Alcaraz to advance from the round-robin stage at the season-ending event for the top eight players.

“I didn't feel well on court,” Alcaraz said. “A few days before coming here, I got sick at home. ... This morning, I feel uncomfortable in the stomach.”

In the same group, Alexander Zverev began his bid for a third ATP Finals title by beating Andrey Rublev 6-4, 6-4.

The top two finishers from each group advance to the semifinals.

On Sunday, top-ranked Jannik Sinner and U.S. Open finalist Taylor Fritz beat Alex de Minaur and Daniil Medvedev, respectively, in the other group.

Alcaraz won the French Open and Wimbledon to boost his career total to four Grand Slam titles. But he also struggled at the Paris Masters recently and dropped behind Zverev to No. 3 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 about it. But I think it's about time, about getting experience. ... A lot of players are better than me on indoor court.”

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

“I'm not full of confidence,” Ruud said. “A match like today might be able to change it.”

Alcaraz was broken when he served for the second set at 5-4.

Alcaraz led 25-16 in winners but committed 26 unforced errors to Ruud's 10.

Ruud converted all four of his break points while Alcaraz was just one of six in that category.

“I'm tired mentally," Alcaraz said. "Obviously a lot of matches, really tight schedule, really demanding year with not too many days off.”

Zverev rolling after Paris Masters title

Zverev, who won the finals in 2018 in London and 2021 in Turin, extended his winning streak to six matches following a title run at the Paris Masters.

Rublev is making his fifth consecutive appearance at the finals but hasn't won a match at the event since 2022.

Zverev served nine aces and dropped only 10 points on his serve in 10 service games.

“He served wide 220 (kph) on the deuce side,” Rublev said. "Normally everybody serves slice and it's 190. He find(s) a way to serve 220 wide on the deuce side. It's not easy.

“Even if you catch the direction, when the flat serve comes 227, slice serve comes 219, 215, it's tough to just see the ball.”

Sinner receives no. 1 trophy

Before the Zverev-Rublev match, Sinnner received the trophy for finishing the year at No. 1 in the rankings.

“There's no place better to celebrate this trophy,” Sinner said as his mom, Siglinde, looked on in tears. “Thanks for all of your support.”

Sinner is playing at home for the first time since it was announced before his U.S. Open title that he tested positive in two separate drug tests this year.

A decision to clear Sinner of wrongdoing was appealed by the World Anti-Doping Agency in September. A final ruling in the case is expected early next year.