Rafael Nadal said Monday that he has tested positive for COVID-19 after returning from a tennis exhibition tournament in Abu Dhabi last week, leaving his participation in the Australian Open next month up in the air. The 20-time Grand Slam champion said on Twitter that he took a PCR test upon returning to Spain and got a positive result. Nadal said he tested negative when he was in Kuwait and Abu Dhabi and last tested negative on Friday. "I am experiencing some unpleasant moments but I hope that I will improve little by little. I am now confined at home and have informed those people who have been in contact with me of the result," he wrote in Spanish. Before his positive result, Nadal told Tennis World USA he was fully vaccinated and planned on playing in the Australian Open in January. However, the positive test has now complicated his comeback. "As a result of the situation I have to have complete flexibility with my calendar and will be analyzing my options depending on my progress. I will keep you informed of any decision about my future tournaments," Nadal added. The 35-year-old played in the Mubadala World Tennis Championship, his first tournament appearance since August. He later announced he would not compete in the U.S. Open and miss the rest of the 2021 season because of a chronic foot injury. Last year, he also dropped out of the Tokyo Olympics and Wimbledon. Despite dealing with a myriad of injuries throughout his career, Nadal is one of the greatest tennis players of all time. He is currently tied with men's No. 1 ranked player Novak Djokovic and Roger Federer for the most men's Grand Slam tournaments won at 20. 