Lyon, who drew 1-1 at Evian on Friday, are on seven points from three games, ahead of Valenciennes on goals scored. Toulouse are third, also on seven points, after they won 1-0 at Nancy thanks to an early goal from Moussa Sissoko.
Mamadou Samassa headed Valenciennes in front on 18 minutes from a Gael Danic free-kick.
Ajaccio were reduced to 10 men before the break when Jean-Baptiste Pierrazzi was shown a straight red card for a rough challenge on Danic making the task in hand easier for Valenciennes
They doubled their lead when Loris Nery scored after a superb solitary effort from the left on 56 minutes and wrapped the match up 10 minutes before the end thanks to a David Oberhauser own goal.
In the heat of the Cote d'Azur, Lille took the lead after 36 minutes thanks to France international winger Dimitri Payet but had to come back from a goal down in the second half after defender Nemanja Pejcinovic and forward Eric Bautheac scored on either side of the interval to put the hosts ahead.
Lille, who revamped their starting line-up following their Champions League playoff first leg defeat at Copenhagen on Tuesday, levelled on 59 minutes when full back Djibril Sidibe shot past Joris Delle to mark his Ligue 1 debut.
Nice, who failed to score in their first two games this season, had several chances for the winner, only to see a brilliant Landreau deny them.
"Landreau was everywhere. He clearly won a point for this team," Nice's coach Claude Puel told reporters.
The keeper, who had already kept out striker Alexy Bosetti when one-on-one midway through the first half, parried a powerful Renato Civelli header following a corner kick in the 66th minute.
Six minutes later, in full view of France's coach Didier Deschamps, he made a reflex save to stop a diverted Bautheac free-kick and save a point for Lille.
"This was my best save tonight," said Landreau. "It is always good to make such a performance when the national coach is there."
Promoted Bastia, who had a chance to grab the top spot, suffered their first loss this season 3-2 at Stade Rennes.
Bastia twice cancelled out Rennes' lead before France midfielder Yann Mvila scored a lucky winner six minutes from time as his powerful kick was twice diverted before entering the goal.
A stoppage-time own goal gave former European Cup finalists Stade Reims, promoted last May, their first win of the season, 1-0 against Sochaux.
Sochaux are one of two teams without a point along with Saint-Etienne, who host Stade Brest on Sunday.
Champions Montpellier, who have only one point, host Marseille. Paris St Germain entertain Bordeaux but will be without new signing Thiago Silva, who is not fit.