Novak Djokovic has secretly been battling an undisclosed health problem for "many years".

The 39-year-old tennis star made the admission after falling to his hands and knees and appearing to vomit during a shock defeat at the Cincinnati Open, with the 24-time Grand Slam champion becoming visibly unwell during his second-round match against Argentina's Thiago Agustín Tirante in Mason, Ohio, on Saturday (15.08.26.)

Novak won the opening set comfortably but struggled as the match progressed in hot and humid conditions, required medical attention and eventually lost 2-6, 6-4, 6-4 in his first competitive appearance since reaching the Wimbledon semi-finals last month.

Speaking to reporters afterwards, Novak said: "First and foremost, I congratulate my opponent today. Honestly, it hasn't been a pleasant match for me. That's how I've felt. It hasn't been the first time, and it won't be the last. That's just how it is."

The Serbian fell to his knees during the second set and was subsequently seen with a towel held to his face.

He then sat on his bench, removed his shirt and placed ice on his head while receiving treatment from medical staff.

Novak said the weather itself was not responsible for what happened.

He added: "It's because of a health issue I have. I've been dealing with it for many years now. It causes me a lot of problems, especially when there's a lot of humidity and heat."

Novak has not publicly identified the condition.

He added: "I did anticipate it (would be humid), but there are all these things, like nerves and everything involved that make it worse, and that's what happened."

The defeat was another setback in Novak's increasingly selective schedule as he continues his pursuit of an unprecedented 25th Grand Slam singles title.

Cincinnati was only his third tournament outside the Grand Slams this season. He reached the Australian Open final in February, losing to Carlos Alcaraz before later reaching the Wimbledon semi-finals, where Jannik Sinner defeated him in straight sets.

Novak has already established one of the most successful records in tennis history.

His 24 major singles titles include 10 Australian Opens, seven Wimbledon championships, four US Opens and three French Opens, while he has also spent more weeks ranked world No 1 than any other player in the history of the ATP rankings.

He completed another major career ambition at the Paris Olympics in 2024, defeating Carlos in the men's singles final to win his first Olympic gold medal.

His latest physical difficulties come shortly before the US Open, which begins in New York on August 30. Novak is attempting to add another major title to the record he shares with Margaret Court while competing against a younger generation headed by Carlos and Jannik.

The Cincinnati defeat also prompted Novak to cast doubt over whether he will return to the tournament.

Asked about competing there in 2027, he said: "It seems more likely that I won't be back next year."

Novak added: "But let's see what the future holds. I hope I do."

For Thiago, who is ranked world No 50, the victory represented the biggest result of his career.

He said: "I think I really did a good job inside the court. I managed very well the nerves of playing against a legend like Novak."

Thiago added: "Thinking about my family and my friends that are in Argentina watching me, they are really happy to watch me on TV. They are in Argentina, but they are in my head as well."

Thiago advanced to face Spain's Martin Landaluce in the third round, while Novak now has two weeks to prepare for his latest attempt to win a record 25th major at Flushing Meadows.