World No. 1 Novak Djokovic lost one of his two opening singles matches on Saturday at the charity tournament he organized, and his day got worse when news broke that one of the event's four legs has been scrapped due to COVID-19 concerns.

Shortly after beating fellow Serbian Viktor Troicki in his opening match, Djokovic said he was disappointed that the Adria Tour's third leg in Montenegro, scheduled for June 27-28, had been called off.

"I am so sad for the Montenegro leg," he told the Balkan nation's Sport Klub television in a courtside interview in front of a capacity 4,000 crowd.

"We tried everything, but the borders are still not open and we don't have time to organize it all."

With 12,251 confirmed cases and 253 deaths related to COVID-19, Serbia is among the countries whose residents are not allowed to travel to its southern neighbor Montenegro.

Earlier on Saturday, Djokovic's younger brother Djordje and the Adria Tour director released a statement saying the event's board had decided to scrap the Montenegro leg.

Asked at a news conference after his defeat by Filip Krajinovic of Serbia whether there was an alternative host nation able to replace Montenegro in a fortnight, Djokovic said:

"We are looking at several alternative options and locations which I don't want to reveal at this point.

"We are running out of time so we will know where we stand either tomorrow or the day after."

The second leg will be held in Croatia's coastal resort of Zadar on June 20-21 and the final one in Bosnia's Banja Luka on July 3-4.

The tournament, organized by Djokovic while international tennis remains suspended due to the COVID-19 pandemic, has drawn some household names and adopted a more streamlined format than the main tour events.

Sets have been slashed to the best of seven games, with eight players competing in two pools on a round-robin basis.

Having beaten Troicki in straight sets, Djokovic lost 4-2, 2-4, 1-4 to Krajinovic.

Second seed and Austrian world No. 3 Dominic Thiem beat Damir Dzumhur in minutes as the Bosnian retired in the opening set with a hamstring injury, but had to save a match point in his 1-4, 4-1, 4-3 win over Dusan Lajovic.

Bulgarian Grigor Dimitrov beat Lajovic in three sets but was stunned 4-1, 4-3 by home crowd favorite Nikola Milojevic – drafted in to replace Dzumhur.

World No. 7 Alexander Zverev beat Krajinovic 0-4, 4-3, 4-3 and was playing Troicki in a late match starting after midnight local time.