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.
It is played in a round-robin format, with the two group leaders competing in a final on Sunday.
Players need four games to win a set, with a tie-break at 3-3, and matches are best of three sets.
Borna Coric (Cro) beat Grigor Dimitrov (Bul) 4-1 4-1
Danilo Petrovic (Srb) beat Alexander Zverev (Ger) 4-3 (7-2) 4-3 (7-1)
Novak Djokovic (Srb) beat Pedja Krstin (Srb) 4-3 (7-3) 4-1
Andrey Rublev (Rus) beat Marin Cilic (Cro) 4-3 (7-5) 2-4 4-1
Nino Serdarusic (replaced Dimitrov) (Cro) beat Pedja Krstin (Srb) 4-3 (7-5) 4-3 (7-1)
Andrey Rublev (Rus) beat Danilo Petrovic (Srb) 4-2 4-1
Novak Djokovic (Srb) beat Borna Coric (Cro) 4-1 4-3 (7-1)
Alexander Zverev (Ger) beat Marin Cilic (Cro) 4-3 0-4 4-3 (7-1)
A number of players are also competing in the Ultimate Tennis Showdown, an event created by Serena Williams’ coach Patrick Mouratoglou in Nice, France.
Saturday’s matches included Richard Gasquet beating Dominic Thiem, Feliciano Lopez winning against Matteo Berrettini and a Stefanos Tsitsipas victory over Dustin Brown.