LONDON - Croatia’s Borna Coric became the second player from Novak Djokovic’s Adria Tour exhibition tournament to test positive for COVID-19, raising serious concerns for tennis governing bodies in their bid to restart the sport after a lengthy shutdown. The professional circuit was halted in early March as nations closed borders and imposed lockdowns to contain the spread of the virus. Last week the men’s ATP and the WTA, which runs women’s events, issued revised calendars for the resumption of the circuit from August while organisers of the U.S. Open said the Grand Slam will be staged without fans as scheduled from Aug. 31 to Sept. 13 in New York. Those plans, however, could need to be redrawn with Coric’s positive test following that of Grigor Dimitrov, who said on Sunday he had contracted the novel coronavirus, highlighting the risks of athletes from different countries mixing without adhering to social distancing norms. “I want to make sure anyone who has been in contact with me during the last few days get tested,” the 23-year-old said. “I am really sorry for any harm I might have caused. I’m feeling well and don’t have any symptoms.” The ATP and the WTA did not respond to request for comments. Organisers of the Adria Tour could not be reached for comment. Bulgarian Dimitrov flew in to play in Djokovic’s event in Belgrade and the second leg in the Croatian coastal resort of Zadar as did other top names like Dominic Thiem of Austria and Germany’s Alexander Zverev.