Serbia is now 3-1 and Ukraine 2-2. Serbia plays France and Ukraine meets Lithuania in their final second-round games.
Jeter hit a 3-pointer to end the third quarter with a nine-point Ukraine lead, which increased to 11 at the start of the fourth.
With Serbia trying to close the gap, Jeter sank two consecutive 3-pointers, made a jumper and stole the ball before making one of two free throws. That left Ukraine up by eight with 1:05 on the clock and although Nemanja Bjelica made a 3, Ukraine held on to win.
"We knew that if we wanted to win we had to play our best game at both ends of the court," Ukraine coach Mike Fratello said. "Today we had the energy and the effort and we came back from the embarrassment against Latvia."
NBA center Jonas Valanciunas scored 12 points and grabbed 10 rebounds to lead Lithuania, which improved to 3-1 in the second round and looks likely to clinch one of the top four places in its group.
"We used our big guys real well and that was the key," said Lithuania power forward Mindaugas Kuzminskas, who added 12 points and eight rebounds.
Jonathan Tabu led Belgium with 15 points.
Lithuania pulled down 13 more rebounds than Belgium and scored 40 points in the paint against 22 for the Belgians.
"We missed some open shots at the beginning but we knew the scoring would come," Lithuania coach Jonas Kazlauskas said.
Serbia paid the price for 11 turnovers and for missing 12 of 24 free throws. Ukraine scored 15 points off Serbia's turnovers.
"We were fortunate. Things happen on the court, like they drop the ball at half court, that you can't explain," Fratello said.
Bjelica, who led Serbia with 22 points, 10 rebounds and four assists, acknowledged that his team had underestimated Ukraine after its big loss to Latvia.
"And Jeter gave us a lot of problems, he hit a lot of easy baskets," Bjelica said.
Serbia coach Duda Ivkovic said his team "lost confidence and failed to react in a proper way."
"To miss 12 free throws is unbelievable. We need games to gain experience and we must not underestimate anyone," the veteran coach said.
Serbia is the youngest team in the tournament, with nine players making their debut.