World number 19 Grigor Dimitrov, who played in both of Novak Djokovic’s recent Adria Tour events, has tested positive for coronavirus.
“I am so sorry for any harm I might have caused,” he said.
Sunday’s final, which would have featured world number one Djokovic, has been cancelled.
Dimitrov, who also played in the first event in Belgrade last week., wrote on Instagram: “I tested positive back in Monaco for Covid-19. I want to make sure anyone who has been in contact with me during these past days gets tested and takes the necessary precautions.
“I am back home now and recovering. Thanks for your support and please stay safe and healthy.”
The tournaments were set up by world number one Djokovic to help players get back to match fitness after several weeks without competition because of the coronavirus pandemic. Several top-10 players have been competing including Djokovic, Alexander Zverev and Dominic Thiem.
The first event in Serbia attracted attracted 4,000 fans, while several thousand supporters have attended in Zadar.
“We have just been informed that Grigor Dimitrov has tested positive for coronavirus and we will have to call off the final of the tournament in Zadar,” Djokovic’s coach and event director Goran Ivanisevic told fans courtside.
There have been no professional tennis events organised by the main men’s and women’s tours since March because of the coronavirus pandemic.