Jonny Wilkinson overcame a nervous display but still kicked all of Toulon's points in a gritty 15-9 win over struggling Agen on Saturday as his team maintained a seven-point lead in the French Top 14.
Despite missing three kicks at goal Wilkinson was successful with four penalties and a second-half drop-goal with South African fly-half Conrad Barnard successful with three penalties for Agen.
"It was a small match but an important victory," said Toulon coach Pierre Mignoni.
"It was essential to get the four points and consolidate our first place. The title of 'autumn champions' is not important, we have won nothing."
Two-time defending champions Toulouse slipped to their fifth defeat in all competitions this season as Grenoble beat them 15-6.
It was the first time in four-and-a-half years that the four-time European champions had suffered two successive defeats, having lost to Welsh side Ospreys in their European Cup clash last Saturday.
Former Toulouse player Valentin Courrent kicked all his side's points in what was their eighth win -- their seventh at home and extending their unbeaten stretch there to 22 -- of a successful return to the elite league while also denying Toulouse a defensive bonus point.
Toulouse, whose five defeats have all been away from home, are nine points adrift of Toulon while 2010 champions Clermont are second -- two points ahead of Toulouse and seven behind Toulon -- having beaten Bordeaux-Begles 28-24 on Friday.
Grenoble coach Sylvain Begon said that victory had been made possible because the players had listened to what his strategy was and replicated it on the pitch.
"The players not only followed the gameplan but they once again showed their spirit is exemplary," said the 40-year-old.
"It is vital not to leave space for Toulouse to attack through. We never panicked, even when we were at one point being dominated by them. It is crucial we continue to preserve this momentum for as long as possible."
Toulouse wing Vincent Clerc conceded that Grenoble had been hungrier for victory.
"There is little to say," said the France star. "Grenoble were without doubt more aggressive, performing better than us.
"In difficult conditions, it was decided on the little things and Grenoble were more committed than us."
Toulouse were forced into recalling hooker William Servat for his fifth start this season despite his retirement at the end of last season after 18-year-old Christopher Toklofua was ruled out shortly before the kick-off because of a hip injury.
It was Grenoble, though, who opened the scoring through an eighth minute penalty from Courrent after Yannick Nyanga had not released Jonathan Best in the tackle.
However Toulouse levelled two minutes later through 21-year-old scrum-half Sebastien Bezy.
Courrent, though, was having a storming start to the game and another piece of delightful positional kicking by the 30-year-old led to a penalty for the hosts, which he converted in the 16th minute.
Bezy, who had been preferred by Guy Noves to Australian star Luke Burgess for the starting spot, again levelled the game but Courrent, who made 33 appearances for Toulouse from 2006-08, then kicked two in a row before edging the hosts three points ahead on the stroke of half-time.
He extended the lead to 12-6 shortly after the restart while Bezy missed one and with under 10 minutes remaining the hosts moved to 15-6 ahead with a superb long range penalty by Courrent.