Cape Town - Fullback Joe Pietersen celebrated his return to the Super 14 for the first time since the opening round with two tries helping the Stormers beat the Force 25-24 on Saturday.
The Force's slim semi-final hopes were extinguished despite the superb efforts of flyhalf Matt Giteau who contributed 19 points from two tries, three conversions and a penalty.
The Australians had to win to stay alive ahead of the final round of group matches next weekend, but the one point they earned left them six points adrift of fourth position.
It was the 12th-placed Stormers's fourth victory of a season that has been disrupted by a long injury list.
Each side scored three tries although the Stormers should have had four.
Centre Dylan des Fountain produced the howler of the season by dropping the ball after he had crossed the tryline with no defender in sight.
Pietersen opened the scoring with a try in the second minute when he scythed through a gap, chipped ahead and collected for a stunning touchdown.
He then sealed the win with another try in the 71st minute which he started with a break before feeding wing Gcobani Bobo. A stumbling Bobo fed the ball back inside and Pietersen was on hand.
Giteau scored his two tries after the interval, the second coming five minutes from the end to reduce to the deficit to a point and ensure a nail-biting finish for the home supporters.
Stormers flyhalf Willem de Waal and Force centre James O'Connor produced the other tries.