The Cheetahs chose the wrong day to give their worst Super 15 performance of the season as they slumped 28-3 Saturday against fellow South Africans Western Stormers.

A win would have left the Bloemfontein-based surprise packets of this southern hemisphere championship season needing just one point against New Zealand visitors the Blues next weekend to seal a top-six finish.

The teams occupying the first six places on the final standings in two weeks' time advance to the knockout phase and while the Cheetahs are still contenders, a home loss to the Blues could spell disaster.

Defending champions the Chiefs (61 points) of New Zealand, Northern Bulls (59) of South Africa and ACT Brumbies (59) of Australia occupy the top three places reserved for the conference leaders.

Then come the Queensland Reds (54) of Australia, record seven-time champions the Crusaders (51) of New Zealand and the Cheetahs (46) held on to sixth spot despite a woeful showing at chilly Newlands.

Apart from the Blues (44), South African sides the Sharks and Stormers (42 each) still have a mathematical chance of making the mid-July play-offs.

But the weekend results mean the New South Wales Warathas (44) of Australia and the Hurricanes (39) of New Zealand are out of the reckoning as they cannot surpass 49 points and a last-round bye will take the Cheetahs to 50.

The Stormers brushed off the absence of unfit centre and skipper Jean de Villiers to dominate the game and full-back Joe Pietersen contributed 13 points from two conversions and three penalties.

That tally lifted him to joint third with Beauden Barrett of the Hurricanes on the championship scorers' chart behind runaway pacesetter Morne Steyn of the Bulls and Christian Lealiifano of the Brumbies.

A home team that excelled at scrum time also scored three tries from stand-in skipper Deon Fourie, fellow loose forward Siya Kolisi and 2007 World Rugby Player of the Year and left-wing Bryan Habana.

However, the Stormers failed in a late bid for a bonus-point fourth try as they knocked on the ball after winning a line-out five metres from the Cheetahs try-line.

The Cheetahs were unrecognisable from the side that entered the match with nine victories this season apart from the traditional white and orange strip, and the 13-minute lead from a penalty by fly-half Riaan Smit raised false hopes.

Pietersen levelled from a penalty on 32 minutes and converted a try by Fourie, who dotted down after a driving maul off a line-out, to give the Stormers a 10-3 half-time advantage.

Smit fluffed a second penalty chance when his team trailed by 13 points and any lingering doubts about the outcome vanished on 66 minutes as a clever inside pass from scrum-half Louis Schreuder set up Kolisi to score.