Paris St-Germain's 2-1 defeat at Stade Rennes in their French league opener brought Vahid Halilhodzic's overconfident team down to earth and confirmed that they must not take anything for granted in the title race.
They were the only team among the big names to lose their first match as champions Lyon and title challengers Marseille and Monaco began the new campaign with victories on the opening weekend.
PSG's loss was a setback but Halilhodzic hopes it will remind his players to adopt a more modest approach.
"I told the players before the match we were not totally ready. We were overestimating our strength, and unfortunately I was right," he said. "We played some very bad football tonight and proved much too naive."
Stade Rennes scored two fine goals in two minutes midway through the first half thanks to Olivier Sorlin and Alexander Frei as they put the visitors in trouble. PSG were lucky to get back into the game with a penalty, converted by Portugal international Pedro Pauleta, five minutes before the break but were physically unable to move up a gear in the second half as Rennes hung on for the victory.
Lyon, who had not won their opening match since 1981, had a 1-0 win at Nice thanks to striker Giovanni Elber on Friday.
Champions' League finalists Monaco held off determined hosts St Etienne for 89 minutes after taking the lead through Uruguay's Ernesto Javier Chevanton in the first minute. Stubborn Marseille snatched a 1-0 victory at home to Girondins Bordeaux as midfielder Laurent Battles slotted in the winner four minutes into injury time.
Bayern Munich began their bid to claim back the Bundesliga title with a 2-0 victory at Hamburg SV on Saturday to give new coach Felix Magath a winning start. Germany's Michael Ballack opened the scoring and fellow midfielder Sebastian Deisler added the second with a free-kick 19 minutes from time.
Champions Werder Bremen kicked off the season on Friday with a 1-0 home victory over Schalke 04. The big surprises of Saturday were Borussia Dortmund's 2-1 home defeat by VfL Wolfsburg, and Hertha Berlin surrendering a two-goal lead to draw 2-2 against VfL Bochum.