Bayern Munich coach Juergen Klinsmann had said his team was ready to claim the lead, but Hertha won thanks to goals from Voronin near the end of the first half and with 13 minutes from time.
"We are disappointed and angry," Klinsmann said. "We wanted to get three points here. But now we just have to continue our good run that was interrupted by [losses to] Hamburg and Berlin."
Only four points separate the top five teams:
Hertha Berlin 40
Bayern Munich 38
Bayer Leverkusen 36
Hamburg moved into third place with a 2-0 win over Arminia Bielefeld on Sunday on goals by Germany winger Piotr Trochowski and Peruvian forward Jose Guerrero.
"We made too many errors, which is not typical of us," said Hoffenheim coach Ralf Rangnick. "We conceded two goals in the first ten minutes. There were a few key passages of play for us, above all the goals just before and after halftime. It is crucial now to see how the team responds in our next game against Stuttgart."