Petkovic has guided his side to the Coppa Italia final and the quarter-finals of the Europa League.
Ahead of tomorrow night's Rome derby, Lazio are sixth in Serie A, with Petkovic convinced that his side are still capable of achieving big things.
He told reporters at a pre-match press conference: "In July we were not even considered to be candidates for European qualification. Nobody thought we could even compete.
"I am very proud of the way our season is going. We are in good shape at the moment and I see the lads calm in training. We've played several important matches this term and that can help us to prepare, even if the European journeys are not helping our fitness levels.
"In this game every detail can be decisive. Lazio must remain solid and maintain that team spirit if we want to achieve our objective. We must play our usual game [tomorrow], which is to put Roma under pressure."
The Rome giants face a crucial week in their fight on three fronts, with Monday's derby followed three days later by a Europe League quarter-final second leg at home to Fenerbahce.