Welsh full-back Leigh Halfpenny scored the crucial late try in a personal haul of 21 points as Toulon reached the French Top 14 semi-finals with a 26-22 play-off win over Castres on Friday.
Toulon will face La Rochelle in Marseille next Friday for a place in the final, but they did it the hard way being reduced to 13 men at one stage in the second half of a bruising encounter.
Trailing 16-19 with 15 minutes left, Halfpenny was freed by two-time World Cup winner Ma'a Nonu to score in the corner.
The Welsh star knocked over the tricky conversion to confirm a lead his team never surrendered.
Three-time European champions Toulon were 3-0 ahead after just two minutes thanks to a penalty from Halfpenny.
But Argentine fly-half Benjamin Urdapilleta levelled just moments later with his trusty left boot.
In a gruelling, physical clash, Toulon grabbed the only try of the first period on the half-hour mark.
Scrum-half Sebastien Tillous-Borde made the crucial break off the back of the scrum, freeing winger Drew Mitchell.
The Wallaby star cleverly switched direction with a smart step inside, fed Francois Trinh-Duc who released prop Laurent Delboubles to barrel over under the posts. Halfpenny popped over the conversion for 10-3.
The Welshman was then penalised for not releasing the ball and Urdapilleta cut the deficit for Castres to 10-6.
Both kickers hit another penalty each as Toulon went into the break at 13-9 ahead.
The game boiled over early in the second half when Toulon's Australian No8 Liam Gill was yellow-carded for a dangerous tackle on Rory Kockott.
Urdapilleta slotted over the penalty for 12-13 before Castres took the lead on the night for the first time when a penalty try was awarded for Mitchell's deliberate knock-on.
The 33-year-old Mitchell was yellow-carded, leaving his team temporarily down to 13 men while Urdapiletta hit the conversion for 19-13.
Halfpenny cut the gap to 16-19 with another penalty before the Welshman played the key role in his team's victory.
Veteran All Blacks centre Nonu's slick handling allowed the supporting Halfpenny to slip into the corner for the try which he converted for 23-19.
He knocked over an insurance penalty for 26-19 and Urdapilleta's late penalty was just a consolation for the 2013 champions.
The second opening round play-off takes place on Saturday when Montpellier face Dan Carter's Racing 92.
The winner will face European Champions Cup runners-up Clermont for a place in the final.