Stefanos Tsitsipas battled back from a 0-2 deficit in the third set to beat Dusan Lajovic, 4-6, 6-3, 6-4 in two hours.
Lajovic was crowned this year’s Croatia Open Umag Champion.
Tsitsipas is currently sixth in the 2019 ATP Race To London.
21-year-old Tsitsipas is now 39-20 on the season that includes titles at the Open 13 Provence in Marseille and the Millennium Estoril Open.
He will now play Georgian Nikoloz Basilashvili, who was a 6-4, 7-6(3) victor over Guido Pella of Argentina in one hour and 42 minutes.