Novak Djokovic lost one of his two opening matches at the Adria Tour, as elite men's tennis returned for the first time since being suspended due to the coronavirus pandemic.
The world number one, 33, cruised past fellow Serb Viktor Troicki 4-1 4-1 in Belgrade in just 34 minutes.
He then lost 4-2 2-4 1-4 to world number 32 Filip Krajinovic.
Djokovic set up the tournament to help players get back to match fitness after several weeks of no competition.
The cross-Balkan event has a reduced match format and attracted 4,000 fans.
But the tournament's third leg, scheduled to take place in Montenegro, has been cancelled because Serbian citizens are banned from entering the country amid the coronavirus pandemic.