Rafael Nadal tests positive after a tournament in Abu Dhabi.

Rafael Nadal, the Spanish tennis star, announced that he had tested positive for the coronavirus in Spain on Monday after returning from an exhibition tournament in Abu Dhabi.

Nadal, a former No. 1 and one of the greatest players in the sport’s long history, shares the men’s record with 20 Grand Slam singles titles. But because he missed most of the second half of the 2021 tennis season because of a chronic problem with his left foot, he played only 29 matches this season.

His positive test, announced Monday, casts doubt on whether he will return to the circuit next month for the Australian Open, the first Grand Slam tournament of the year.

“I am having some unpleasant moments but am counting on feeling better bit by bit,” he said in a post on Twitter in Spanish on Monday. He added: “As a consequence of the situation, I have to maintain total flexibility with my schedule, and I will analyze my options depending on how my situation evolves.”

Nadal said he had taken a P.C.R. test after returning to Spain from Abu Dhabi and was now confined to his home in Manacor on the Spanish island of Majorca. He said he had informed all those with whom he had been in close contact about his test result, including his wife Maribel, his father Sebastián, his fitness trainer Rafael Maymó and two of Nadal’s coaches: Carlos Moyà and Marc López, according to El País, a Spanish newspaper.

The list of close contacts also presumably included Nadal’s two opponents in the Abu Dhabi exhibition: Andy Murray and Denis Shapovalov, both of whom defeated Nadal.

Juan Carlos, the former king of Spain, attended Nadal’s match with Murray on Friday and met on Saturday with Nadal and Moyà, according to the Spanish newspaper El Mundo.

Nadal is the latest tennis star to contract the coronavirus. No. 1 Novak Djokovic tested positive for it in 2020. No. 2 Daniil Medvedev did so earlier this year, as did Aryna Sabalenka, No. 2 in the women’s singles rankings, and Coco Gauff, the rising American teenager who was unvaccinated at the time and was forced to miss the Summer Olympics.

Unlike Djokovic, who has not revealed his vaccination status, Nadal has been an advocate for vaccination and said he would comply with the upcoming Australian Open’s decision to require players to be vaccinated.

“If the people who really know about it say that we need to be vaccinated, who am I to create a different opinion?” he said in Abu Dhabi last week.

Nadal, ranked 6th after his abbreviated 2021 season, had to withdraw from Wimbledon, the Summer Olympics and United States Open this year because of Müller-Weiss syndrome, a congenital foot condition that first troubled him in his teens but which he has long been able to manage with custom insoles and therapy.