Federico Coria and Jeremy Chardy, along with Serbian Pedja Krstin, progressed into the second round in Belgrade on Sunday.
Coria came from a set down to win 4-6 6-2 6-0 in the first-round match in the Serbia Open against ATP Tour debutant Marko Topo.
Chardy overcame Tennys Sandgren 6-4 6-2. The Frenchman started the season well, and managed two semi-final runs, but has been out of form in recent competitions and was knocked out in the first round at the Madrid Masters earlier this month.
Meanwhile, Pedja Krstin made home advantage count against Kwon Soonwoo.
The trio will be joined in the second round by top seed and world number one Novak Djokovic, who received a first-round bye and is in the hunt for his third title in Belgrade.
At the Parma Open, American Tommy Paul reeled off a 7-5 6-4 win over Stefano Travaglia, and Flavio Cobolli beat Marcos Giron 3-6 7-6 (8-6) 7-6 (7-4).
