In this June 11, 2021 file photo, Spain's Rafael Nadal reacts during his men's singles semi-final tennis match against Serbia's Novak Djokovic on Day 13 of The Roland Garros 2021 French Open tennis tournament in Paris. (MARTIN BUREAU / AFP)

Rafa Nadal said on Monday he had tested positive for COVID-19 after returning from his comeback appearance at the Mubadala World Tennis Championship exhibition event in Abu Dhabi last week.

The 20-times Grand Slam champion and former world number one said he was having some "unpleasant moments" but was hoping to feel better soon and would keep people informed about his plans over future tournaments.

The positive test casts further doubt on whether the 35-year-old participates in next month's Australian Open – a tournament he has won once and finished runner-up four times – having already said he could not guarantee he would play

The positive test casts further doubt on whether the 35-year-old participates in next month's Australian Open – a tournament he has won once and finished runner-up four times – having already said he could not guarantee he would play.

ALSO READ: Nadal casts doubt over Australian Open participation

"I wanted to announce that on my return home after playing the Abu Dhabi tournament, I have tested positive for COVID-19 in the PCR test that has been performed on me when I arrived in Spain," Nadal wrote on Twitter.

Nadal said he had tested negative at all times when he was in Kuwait and Abu Dhabi, and had last tested negative on Friday last week.

During his stay in Abu Dhabi, Nadal met former Spanish king Juan Carlos, who tested negative for COVID-19, Spain's Royal Household said on Monday. The monarch, who lives in the United Arab Emirates, is feeling good, the palace added.

Nadal's comeback from injury at the exhibition event – his first tournament appearance since the Citi Open in Washington in August – ended in two defeats by Andy Murray and Denis Shapovalov.

"I am having some unpleasant moments but I hope that I will improve little by little. I am now homebound and have reported the result to those who have been in contact with me," he wrote.

"As a result of the situation I have to have total flexibility with my calendar and I will analyze my options depending on my evolution. I will keep you informed of any decisions about my future tournaments."

Murray was forced to withdraw from the season's opening Grand Slam earlier this year after testing positive for the coronavirus in January.

Spain's Rafael Nadal competes against Britain's Andy Murray during their semi-final match in the Mubadala World Tennis Championship in the Gulf emirate of Abu Dhabi on Dec 17, 2021. (GIUSEPPE CACACE / AFP)

Nadal spent the last four months on the sidelines with a foot injury. He was knocked out at Roland Garros in the semi-finals and was forced to pull out of Wimbledon, the Tokyo Olympics and the US Open.

He also missed the Davis Cup which concluded in Madrid earlier this month and will not feature for Spain at the Jan 1-9 ATP Cup in Sydney.

The Australian Open begins on Jan 17.