Argentina winger Emiliano Boffelli scored on his first start to send Racing 92 to the summit of France's Top 14 with a 23-20 home win over Castres on Friday.
Boffelli crossed in the closing moments as his new side, champions in 2016, moved above La Rochelle and holders Toulouse to top.
Boffelli came off the bench in last weekend's victory over Lyon for his debut for Laurent Travers' men after joining in December but they were without the injured Virimi Vakatawa, Kurtley Beale and Camille Chat, who watched from the stands.
The visitors' coach Mauricio Reggiardo made six changes, all in the pack, from last Saturday's hammering over Montpellier.
Both outfits were missing their France Six Nations contingents with Teddy Thomas and Bernard Le Roux absent for the home side and Anthony Jelonch unavailable for Reggiardo.
At the Paris La Defense Arena, the hosts led 13-3 after 20 minutes as Teddy Iribaren crossed and kicked a conversion and two penalties with Pumas fly-half Benjamin Udrapilleta responding with an effort at the posts for the away side.
Iribaren added a third penalty before Fiji winger Filipo Nakosi crashed over from short range just in added time as Travers' side were 16-13 up at the break.
Castres continued their dominance after the interval and after 15 minutes of pressure 21-year-old prop Wayan De Benedittis crossed for his second try in as many league starts before Urdapilleta converted to make it 20-16.
Boffelli put his outfit back in front with less than 10 minutes remaining as he claimed his first try since featuring for the Jaguares at the Bulls in the Super Rugby.
Substitute hooker Kevin Le Guen swivelled out of a tackle and fed Boffelli with a delicate off-load for the 31-time Test winger to finish unopposed in the corner.
The game finished on a low as former France flanker Wenceslas Lauret was stretchered off at full-time with a suspected concussion.
On Saturday, bottom of the table Agen host Montpellier looking to avoid a record 16th straight defeat to start a league season.
Toulouse and La Rochelle can leapfrog Racing as they face Lyon and Stade Francais respectively.
Toulon's trip to Pau is set to go ahead despite their squad registering two positive Covid-19 tests on Friday.