Kamil Chanas exploded for 28 points as Slask Wroclaw beat previously undefeated Maroussi Honda 74-71 in the EuroCup on Tuesday night.
Chanas, a 20-year-old guard, made four of five shots from three-point range and also sank 10 of 12 free throws.
Slask led 30-29 at half-time and 46-42 after three quarters but then saw Panagiotis Yannakis & Co tied the game at 62-62 at the end of regulation.
The hosts then did enough in the end to clinch the overtime win.
The triumph has left Slask with three victories and two defeats, giving them an excellent chance of reaching Round II.
Hapoel, who have two wins from four games, hosts winless MPC Amsterdam on Wednesday.
"I am very satisfied of our win," Chanas said. "Now I can say we are going to win in Tel Aviv."
Tomasz Jankowski, the Slask coach, said: "Our young players showed their usefulness for the team when the most experienced players have failed.
"We are glad to win the game because it is a big step to qualify for the next round."
Maroussi had already qualified for the next phase of the competition.
Still, Yannakis would have preferred to have stayed undefeated in the EuroCup.
"Our team lost concentration at the end," he said.
"The Wroclaw team was motivated by fans in the arena.
"We were also motivated but in the last five minutes, Slask had clearer minds."