The Frenchwoman, who had slipped down the rankings after 15 consecutive defeats between the end of July and mid-January, seemed on her way up again thanks to some recent landmark wins.
It appeared that a year after reaching the quarter-finals, she could make light work of German Petkovic, a former semi-finalist at Roland Garros. However, two consecutive double faults as she had three set points in the opening set's tiebreak threw her off balance.
Mladenovic, who recently beat former French Open finalist Sam Stosur, never recovered and allowed world number 107 Petkovic to claim only her third victory on the main tour this year -- also her fifth in as many encounters between the two.
Mladenovic started confidently on the Suzanne Lenglen court, where last year she beat then defending champion Garbine Muguruza to reach the last eight, opening up a 2-0 lead.
But Petkovic broke back and forced a tiebreak, in which the Frenchwoman, seeded 29th, went 6-3 ahead.
The mercury rose in the stadium with the crowd chanting 'Kiki, Kiki!' to support a player who set their hearts on fire last year.
Mladenovic, however, collapsed, serving two consecutive double faults on set points and eventually losing the tiebreak 12-10 as she netted a forehand.
Petkovic suffered late jitters as she served a double fault on match point, but on her fourth attempt, Mladenovic returned long.
Petkovic next faces Sweden's Johanna Larsson or American Bethanie Mattek-Sands.
(Reporting by Julien Pretot; Editing by Christian Radnedge)
