The Biancocelesti played out a thrilling 3-3 draw against Borussia Monchengladbach in the Europa League on Thursday night and now face Siena in Serie A on Monday.
However, despite the packed fixture list, Petkovic has said that they must not use it as an "alibi" for any poor results in the league.
"We have no alibis. Europe can cause tiredness, as we saw with Juventus losing to Roma last night, but we wanted the bicycle, so now let's get pedalling," Petkovic told reporters in his press conference.
"We are close to the levels we were at a few weeks ago, so I am not worried. Are we thinking about the Scudetto? No, if anything we're looking over our shoulder, seeing as from the start of the season people treat us as if we're a little club.
"We must not fail. This game will require us to play at 120% of our potential. It's true we have been a little unlucky recently, but fortune favours the brave."
Lazio currently sit in fourth place in Serie A.