Swansea - Sam Davies came off the bench to lift Ospreys to a gritty 21-13 comeback win over French side Clermont to top Pool 2 in the European Champions Cup on Friday.
Flyhalf Davies kicked 15 points after the break to put the Welsh region within touching distance of the quarter-finals.
Clermont, last year's runners-up, had held the advantage until the final 15 minutes but paid for ill-discipline with their progress now out of their own hands.
"I'm frustrated, angry and disappointed," said Clermont coach Franck Azema.
"We return with zero points. We made a good start to the match, we dominated outrageously up front and were weak behind.
"We were much too ill-disciplined."
Ospreys, looking to become the first Welsh side to qualify for the knock-out stages in four years, overtake Clermont on top of Pool 2 with 16 points.
Clermont have 14 points, with pool rivals Exeter (10) travelling to French club Bordeaux-Begles (6) on Saturday.
To finish first and qualify directly Clermont will have to win at home against Bordeaux-Begles in the final pool game. If Ospreys beat Exeter in nine days' time, they will win the group and secure a quarter-final place.
"I am extremely proud of the players," head coach Steve Tandy said.
"They battled throughout the game, and to come back from 13-6 down against a really experienced Clermont team who have got to how many finals in European rugby, was a real team effort.
"We probably haven't got as many glittering individuals as other European teams have got, but what we have got is a massive team ethic and a massive spirit in the group.
"There are not too many days that I have been prouder of this group, if I am honest.
"It has got to be up there with one of our best performances, but we still haven't achieved what we want to. We have got one more game to go, otherwise it will be all for nothing."
It had looked as if it was going to be a frustrating night for Ospreys as Clermont led by 13-7 at half-time thanks to a Wesley Fofana try on six minutes and two Morgan Parra penalties.
Things got worse for the Welsh when flyhalf star Dan Biggar limped off after 48 minutes. But his replacement Davies took charge and kicked four penalties and a drop-goal -- to add to two earlier Biggar penalties.
Clermont's task was hampered by the sin-binning of Viktor Kolelishvili, for pushing referee Wayne Barnes, and John Ulugia, which left them at a disadvantage for 20 minutes during the game.
The French lacked discipline at key moments, particularly late in the game when trailing 18-13 and pushing for a draw.
Jonathan Davies let the ball slip just metres from the line in the final action as Clermont returned home without even a losing bonus point, and with possibility of missing the quarter-finals for the first time since 2010-2011.