Novak Djokovic suffered his first loss of 2020 as the Adria Tour exhibition event got underway in Belgrade on Saturday.
The 17-time grand slam champion started the event with a 4-1 4-1 victory over fellow Serbian Viktor Troicki.
However, Djokovic was stunned by Filip Krajinovic 2-4 4-2 4-1 in his second match, beaten for the first time this year.
Djokovic was 18-0 on the ATP Tour in 2020, including winning the Australian Open, before the season was suspended due to the coronavirus pandemic.
The 33-year-old launched the Adria Tour last month, although the event scheduled for Montenegro later in June was cancelled.
Alexander Zverev and Dominic Thiem claimed two wins from as many matches on Saturday.