Marseille paid the price for the second-half dismissal of Joey Barton as they slipped to a shock defeat at home to lowly Nancy.
The on-loan midfielder was shown two yellow cards in the space of five minutes to leave OM facing an uphill struggle at the Stade Velodrome.
And an own goal from Andre Ayew capped off a miserable evening for the title hopefuls, who remain six points behind Ligue 1 leaders Paris St Germain.
It marked only the third win of the season for Nancy, lifting them off the foot of the standings and within four points of safety.
Marseille had won their last eight meetings with the Lorraine club and could have taken an 11th-minute lead tonight, but Andre-Pierre Gignac fired inches past the post from a central position.
Gignac then came to the rescue at the other end of the pitch, clearing Paul Alo'o Efoulou's overhead kick off the line with goalkeeper Steve Mandanda beaten.
Djamel Bakar also came close for the visitors moments later, combining with Romain Grange before firing narrowly wide with his left foot.
And Marseille were breathing another sigh of relief before the interval, with Mandanda pulling off an excellent save to turn away a powerful drive from Alo'o Efoulou.
The Cameroon striker continued to cause problems after the restart and a curling right-footed shot grazed the crossbar on its way over.
Nancy goalkeeper Damien Gregorini had done little of note so far, but his agility was tested after 53 minutes by a free-kick from Mathieu Valbuena which took a wicked deflection.
Things began to go wrong for Marseille on the hour mark when Barton was sent off for a bad tackle from behind on Djamel Bakar, just five minutes after his first booking for a challenge on Vincent Muratori.
With a man advantage, Nancy needed no second invitation although there was a touch of fortune about their goal when it arrived in the 74th minute.
Muratori's corner found Salif Sane, whose header bounced off Ayew's thigh and into the net.
Substitute Jeff Louis could have added another as the clock ticked down but Mandanda was able to match the 15-yard strike.