Northampton ended their Heineken Cup pool with a 13-3 victory over French champions Castres but it took them until the final minute to break down a ferocious visiting defence.
George Pisi scored the only try of a clash where Northampton's power game suffered an outage against Castres' muscle-bound blue line.
The win could still see Northampton reach the quarter-finals as one of the two best runners-up if results go their way, but the Amlin Cup seems more likely.
Castres came to Franklin's Gardens with no chance of reaching the quarter-finals but made just one change to the starting 15 that lost to Leinster, with Daniel Kirkpatrick coming in for Remi Tales at fly-half.
Northampton brought Lee Dickson in at scrum-half with Samoan Fa'atoina Autagavaia making his first start for the club at full-back in place of the injured Tom Collins.
The home side looked for four tries from the start as they knew only five points and pointless defeat for Leinster would see them top pool one. They turned down easy penalty chances in the opening minutes, but a series of scrums did not produce a try.
Northampton were dominant at the scrum and after 27 minutes Castres tighthead Anton Peikrishvili was replaced by Mihaita Lazar and it was not clear that he was injured.
Despite that, Castres were hard in the tackle and showed they meant business with even Courtney Lawes bent double by one hit from Piula Faasalele, plus they were disrupting Northampton's usually efficient lineout.
Castres thought they had taken the lead in the 33rd minute when a quick lineout on halfway saw Remi Grosso scoot under the post, but referee Alain Rolland went to the TMO who spotted they had used a different ball.
Saints could not get into their attacking game, caught in the web-like Castres defence and in a half of just two penalties on the floor, the second one saw Stephen Myler make it 3-0 on the half-time whistle.
Marcel Garvey almost got Castres on the scoreboard, with a superb 30-metre break, but the former Worcester flyer slipped as he looked to step round full-back Autagavaia and as the attack fizzled out, Kirkpatrick put a drop goal attempt well wide. But when they won a penalty on 54 minutes, almost 50 metres out, Rory Kockott levelled the scores.
When Northampton needed a big shove from their scrum it deserted them five metres from the Castres try line, while Christian Day and Tom Wood made breaks on the left flank but twice the pass to George North went astray. And despite all their pressure it looked like they would fall behind when Kockott fired a 50m-plus penalty on target but his kick fell just short.
But with four minutes to go smart play from Kahn Fotuali'i saw the replacement scrum half throw a pass into Mihai Lazar's head and from the offside Myler slotted an easy penalty to make it 6-3.
And, when Kockott was charged down by Pisi in his 22, the Samoan centre gleefully dived onto the bouncing ball to score the first try of the game in the final minute, which Glenn Dickson converted.