Right-wing Aisea Havili scored a hat-trick of tries to take Worcester through to the semi-finals of the European Challenge Cup.

The lead changed hands eight times in a pulsating match but the Tongan eventually put the game beyond Northampton's reach with his third try in the 77th minute.

It was the fourth time the two sides had met in various competitions this season and the honours are now even - but the away side have won on each occasion.

The lead swapped between the sides six times in a spell-binding first half.

With fly-half Carlos Spencer working his own brand of magic from the opening minute, Northampton put Worcester on the back foot and Spencer's one-handed pass launched the attack which resulted in flanker Paul Tupai forcing his way over.

Worcester responded with a lengthy spell of intense pressure which saw full-back Thinus Delport and scrum-half Matt Powell both stopped just short.

They spurned two kickable penalty chances to go for touch and their adventure paid off when the Saints defence was eventually stretched to breaking point for centre Thomas Lombard to run in a 28th-minute try, converted by fly-half Shane Drahm.

Full-back Bruce Reihana's penalty put Northampton back ahead at 8-7 but the lead lasted only a minute before Worcester swept back with a high-speed four-man move which sent Havili in for his first try in the corner.

Two minutes later Reihana struck back for Saints, taking a superbly-timed pass from Spencer to slice through the Worcester defence for a try which he converted himself.

But, in the 39th minute, Northampton's rush defence fell apart alarmingly as the game swung again with Worcester moving the ball at lightning speed to provide Havili with a simple second try.

The lead changed hands for the seventh time after Worcester's replacement hooker Chris Fortey was shown a yellow card only eight minutes after being introduced for the second half.

While he was trudging disconsolately to his 10 minutes in the sin-bin, Northampton took full advantage to drive Tupai over for his second try.

But only four minutes later, Worcester retaliated again. Drahm, pinching a page from Spencer's book of tricks, provided the delightful chip kick and the ball bounced up perfectly for big centre Dale Rasmussen to gratefully collect for the try which put the Warriors back in the ascendancy at 24-20.

The chances continued to come thick and fast and centre John Clarke squandered a gilt-edged one for Northampton, fumbling the ball with the line at his mercy, as he attempted to collect his own incisive kick into space.

A heavy knock to the thigh which left Spencer struggling caused Northampton to lose some of their momentum and, even though they threw everything into attack they were susceptible on the counter.

Havili was denied his hat-trick after skinning Ben Cohen with a kick over the head of the England winger before Drahm consolidated the Worcester lead with a 65th-minute penalty.

But a breakaway attack saw the Tongan race clear for his third try, converted by Drahm, in the 77th minute and although replacement forward Darren Fox replied for Northampton deep into injury time, his try was merely a consolation.