World number one Novak Djokovic secured two wins on the opening day of his second Adria Tour event.
Djokovic beat fellow Serb Pedja Krstin 4-3 (7-3) 4-1 in his first match before seeing off Borna Coric, of Croatia, 4-1 4-3 (7-1).
Alexander Zverev and Marin Cilic both lost their respective opening matches, to Danilo Petrovic and Andrey Rublev.
Zverev then beat Cilic 4-3 0-4 4-3 (7-1) in the day's final match to stay in with a chance of making the final.
The tournament is the second of the series which Djokovic set up to help players get back to match fitness after several weeks without competition because of the coronavirus pandemic.