Twenty-one-time Grand Slam champion Novak Djokovic of Serbia expressed his hope of participating in the upcoming U.S. Open on Instagram on Saturday.

"I am preparing as if I will be allowed to compete, while I await to hear if there is any room for me to travel to US. Fingers crossed!" Djokovic wrote.

The U.S. Open is scheduled to start at the USTA Billie Jean King National Tennis Center on September 11. Though Djokovic's name was included in the entry list of the tournament in early July, he will not necessarily be able to take part because travelers must be fully vaccinated against COVID-19 and provide proof before they can enter the U.S.

Djokovic has not been vaccinated, which cost him a place in the Australian Open in January. Though he may miss the last Grand Slam event this year, the 35-year-old Serbian superstar remains firm on rejecting the vaccine.

Djokovic played at Wimbledon and won the tournament, adding his 21st Grand Slam title, only one behind Rafael Nadal. However, as the All England Lawn Tennis Club and the Association of Tennis Professionals (ATP) disagreed on the ban of Russian and Belarusian players from Wimbledon, participants in the event didn't get any ranking points.

As a result, Djokovic's world ranking dropped to seventh. If he misses the U.S. Open, he may fall out of the top 20, according to senior international tennis correspondent Vuk Brajovic of Serbia.

More than 43,000 people have signed an online petition calling for the USTA to work with the U.S. government to allow Djokovic to compete in the U.S. Open. He expressed gratitude for their support.

"I just wanted to take a moment and say to all of you how grateful I am to see so many messages of support and love from all around the world these days. I wasn't expecting it, and that's why it feels so mindblowing. Just wanted to say THANK YOU. It feels special that people connect with my tennis career in such a loving and supportive way and wish for me to continue to compete," Djokovic posted on Instagram.