LOS ANGELES: Top-ranked Jannik Sinner arrives at the US Open as the defending champion and a leading contender, though his withdrawal from the Cincinnati final on Monday due to illness adds a measure of uncertainty to his title defense.

The Italian looked off from the outset of his match against rival Carlos Alcaraz and, after quickly falling behind 5-0 in the first set, retired from the match, saying he “didn’t feel great” and apologizing to the disappointed fans in Ohio.

Tennis World USA reported that Sinner was battling a high fever and flu symptoms the night before and during his match with Alcaraz.

Sinner also mentioned the punishing heat and humidity during his match against Alcaraz, calling it “one of the hottest tournaments we played.” He then announced that he had withdrawn prior to his US Open mixed doubles match on Tuesday.

How quickly the 24-year-old can bounce back from his illness will have major implications for the final Grand Slam tournament of the year.

Sinner and world No. 2 Alcaraz have separated themselves from the pack, having won the last seven major titles between them.

While Sinner has compiled a stellar 31-4 win-loss record this year, three of those defeats have come at the hands of the Spaniard, who rolls into Flushing Meadows with a ton of momentum and a chance to grab the world No. 1 ranking from Sinner.

Prior to Monday’s final, Sinner looked unstoppable, not dropping a set in his first tournament since winning Wimbledon for his fourth Grand Slam title in July.

“I feel like it was a very positive week,” Sinner, who won the Cincinnati title last year, said on Monday.

“Making finals of a Masters event, it’s always an amazing achievement. So we’ll keep going, keep pushing. I have some points where I need to improve if I want to go far in the US Open. And it was a good test this week, trying to understand where my level is.”

He said his focus was firmly on the biggest stages the sport has to offer.

“I love Grand Slams a lot. These are the main tournaments for my season and for my career,” he said.

“So US Open is going to be a tough tournament, but at the same time, I’m looking forward to it. If I’m ready, physically and mentally, I will be ready to push.

“So now I have a couple of days of recovery, and then we get back to work, and hopefully we’ll be ready.”