World number one Novak Djokovic was brought to tears after he bowed out of his Adria Tour with victory over German Alexander Zverev in front of home fans.
The 4-0 1-4 4-2 win sent fellow Serb Filip Krajinovic into the final.
Djokovic’s event – the first elite men’s tennis since it was suspended by coronavirus – attracted 4,000 fans.
“I am just overwhelmed by emotion because this reminds me of my childhood,” he said as the Belgrade crowd gave him a standing ovation.
“It’s been an emotional few days and I want to thank everyone who made this possible. The important thing after this match is that we have one of our own in the final. I love you all and thank you so much for turning up.”
Djokovic, 33, set up the tournament to help players get back to match fitness after several weeks without competition.
Players were split into two groups of eight and played each other in a round-robin format, with the two group leaders competing in a final later on Sunday.
Players need four games to win a set, with a tie-break at 3-3, and matches are best of three sets.
Djokovic, Krajinovic and Zverev all won two of their matches and five sets each, but Krajinovic had the best games differential.
The 28-year-old world number 32 will face either Austrian Dominic Thiem, ranked third in the world, or Bulgaria’s Grigor Dimitrov in the final.
The next leg takes place in Zadar, Croatia from 20 June but the tournament’s third leg, scheduled for Montenegro, has been cancelled because Serbian citizens are banned from entering the country amid the coronavirus pandemic.
The final event is in Bosnia-Herzegovina in July.
Several top 10 players are also competing in the Ultimate Tennis Showdown, an event created by Serena Williams’ coach Patrick Mouratoglou in Nice, France.
Matches are being held without spectators.
Australian 20-year-old Alexei Popyrin beat France’s Elliot Benchetrit in the opener. Italian world number eight Matteo Berrettini defeated Dustin Brown and Feliciano Lopez, Stefanos Tsitsipas and David Goffin are among those yet to play.
The regular ATP Tour is suspended until at least 31 July.