World No.1 Novak Djokovic had a win and a loss in singles at the Adria Tour charity mini-tour he organised.
His day got worse when news broke that one of the event's four legs in the Balkans has been scrapped due to coronavirus concerns.
Shortly after beating fellow Serbian Viktor Troicki in his opening match on Saturday in Belgrade, Djokovic said he was disappointed that the 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 organise 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 neighbour 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.
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 mini-tour, organised 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.
Matches will be played in Fast4 format 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 Filip Krajinovic.
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 favourite Nikola Milojevic - drafted in to replace Dzumhur.
Alexander Zverev beat Krajinovic 0-4 4-3 4-3 and was playing Troicki in a late match starting after midnight local time.
The third-round pool matches will be played on Sunday afternoon followed by a final between the group winners.