TURIN, Italy — Carlos Alcaraz struggled with a stomach issue and looked 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 on Monday.

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 felt uncomfortable in my stomach.

"I'm tired. I'm tired mentally," said Alcaraz, who won the French Open and Wimbledon titles earlier this year.

"Obviously, a lot of matches, a really tight schedule, a really demanding year with not too many days off, not too many periods for you to rest.

"Some players deal with it better than others."

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 US 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 in the rankings this week to No 3.

"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 courts."

Ruud reached the 2021 semifinals and 2022 championship match in his first two appearances at the Finals, but he has struggled recently, too, losing the 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 that."

The Norwegian had suffered seven defeats in eight matches coming into this tournament.

"This part of the season has not been good for me, but hopefully I've saved some wins for this week and I've started well," said Ruud, the 2022 Finals runner-up.

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

The Spaniard 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 from six.

'Solid' Zverev

Zverev opened his campaign at the Turin tournament on Monday evening by dispatching Rublev 6-4,6-4 in their round-robin opener.

Germany's Zverev had last faced Russian Rublev at the same stage of the Finals in 2023 and won by the same score.

Zverev, a two-time winner of this event in 2018 and 2021, moved to second in the John Newcombe group, behind Ruud.

The 27-year-old Zverev, the recent winner of the Paris Masters 1000, has climbed back to No 2 in the world this season. After an injuryhit 2022, he had briefly dropped out of the top 25 in 2023.

"I was never sure I was going to get back to this level," Zverev said on the court in Turin.

"Second year in a row for me here. I still want to get better.

"I still want to improve on a few things. We'll see how next year goes."

Rublev was held to love in his first three service games.

He won a 13th straight point on serve to open the crucial seventh game, but Zverev then won the next four to break to 15.

Zverev did not allow a break point against his serve all match, and converted immediately both times he had the chance.

"I thought it was a very solid match from my end," said Zverev.

"Andrey is an incredible player, but everyone who plays here is an unbelievable player.

"Against anyone here, you have to play your best to have a chance.

"You have to be solid and mentally strong. I feel like I did that today, I felt like I used my chances quite well and I'm obviously happy with this win."

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 km/h on the deuce side," Rublev said.

"Normally everybody serves slice and it's 190. He finds 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 at 227, slice serve comes 219, 215, it's tough to just see the ball."

Agencies