Bookmakers gave struggling Northampton a 20-point start in the quarter-final against last season's Cup runners-up in San Sebastien, but they got it all wrong in a scrappy game that had nothing for the purists, but plenty for Saints fans.

The sides were locked at nil-all after 39 minutes of poor rugby, and with both backlines showing plenty of enterprise, but no precision, it was left to the boot of Biarritz and France goal-kicker Dimitri Yachvili to edge his side in front, only for Robbie Kydd to stun home fans with a match-winning 60-metre intercept try.

Northampton's opponents in the semi-final, London Wasps, won't be too concerned with a trip to Coventry, but they now know what happens when Saints are underestimated.

The visitors showed their hand in the opening minutes by turning down a shot at goal and opting for the lineout. But, in a sign of things to come, Biarritz stole the throw-in and the visitors were left to rue their decision.

With winger Phillipe Bidabe and scrum-half Yachvili looking dangerous, Saints spent much of the opening half an hour on the back foot. Their cause wasn't helped when inspirational winger Sean Lamont was helped from the field with a shoulder injury.

Tempers boiled over as the match approached halftime, with scrum-halves Yachvili and Mark Robinson going toe to toe after yet another promising Biarritz backline move ended with a poor pass and a knock-on.

The French club eventually gave up on the idea of scoring five points, opting for goal instead, but even that didn't come off as Yachvili's attempt from in front drifted off target.

The little Test scrum-half was given another chance on the stroke of halftime, and made no mistake to give the home side a far-from-impressive 3-0 lead at the break.

Saints fly-half Carlos Spencer should have levelled the scores early in the second half, but his infamously wayward goal-kicking came back to haunt the  visitors and their fans.

A Mark Robinson break should have resulted in a Saints lead, but the move broke down in midfield and the game limped into the final quarter. Northampton began leaking penalties, and Yachvili finally converted one to double his side's score and lead.

But just as the French club pushed for a third score, Saints centre Kydd intercepted a Yachvili pass and ran 60 metres to turn the match on its head. Spencer's conversion gave Saints a shock lead and Biarritz fans were stunned into silence.

Biarritz had the chance to steal it in the dying minutes, but backrower Imanol Harinordoquy spilled a Yachvili cross-kick with plenty of support on hand and the try-line begging to be crossed.

A desperate dropped goal attempt was charged down by a wall of Saints defenders, and Biarritz's final two attacks continued the theme of the day - they were both knocked on.