Racing 92 grabbed top spot in the Top 14 table after a hard-fought 22-20 win over Lyon on Saturday, dislodging Stade Francais.
The two teams from the French capital, who will meet in two weeks' time, are both on 18 points after Stade thrashed Castres 39-16 earlier.
At La Defense Arena, Racing finally got past a dogged Lyon side thanks to a 79th minute try from South African fly-half Tristan Tedder.
Earlier, Stade made it four wins in five games as Lucas Peyresblanques and South African centre Jeremy Ward scored two tries apiece and Sekou Macalou and Rory Kockott also crossed at Stade Jean Bouin.
Either Pau or Toulouse could in theory go top depending on the result of their match on Sunday.
Reigning European champions La Rochelle meanwhile lost for the fourth time in their first five games of the season when they were beaten 19-17 away at Oyonnax.
Irish wing Darren Sweetnam scored Oyonnax's only try of the game and Argentinian Domingo Miotti's boot did the rest, with a conversion and four penalties.
Thierry Paiva's converted try in the 84th minute came too late for La Rochelle after Joel Sclavi had also touched down.
La Rochelle, European champions for the past two seasons, have seven points and sit in 12th place.
Coach Ronan O'Gara said: "Rugby's a combat sport, especially when you go to Oyonnax.
"We were aggressive, but we lacked precision all afternoon."
O'Gara said the team needed a change in fortunes.
"I can't deflect the responsibility onto the players because above all it's mine," the Irishman said.
"We need to bounce back quickly. We're capable of it. Something has to change rapidly, and I hope it's not the coach."
In other games, Clermont were convincing 46-14 winners against Bayonne to move onto 14 points.