Petkovic said that it required Milan going ahead for his side to play at their best but once they did they could easily have gone on to win the game.
"Did we need Milan to take the lead in order for us to react? It shouldn't be that way, but it did look like it," Petkovic told reporters.
"We deserved to win at the end, though. I am not satisfied, as we need to face games with more determination. We allowed Milan the initiative in the first half, but the second was the complete opposite.
"Milan have a lot of quality and when they are able to play to their strengths any team will struggle against them. After the equaliser we improved and should've finished the evening with a victory. We lack that extra 10% of determination away from home to win."
Lazio are seventh after the result, while Milan are 10th.