The Western Force were struck by a virus and then knocked out cold by the Stormers in an embarrassing 51-16 Super Rugby loss in Cape Town on Saturday night.

Force backs Nick Cummins, Gene Fairbanks, Alfie Mafi and Pat Dellit were laid low by a virus on the eve of the match, with Fairbanks hit the hardest and unable to take his spot.

But coach Richard Graham refused to use the untimely sickness as an excuse, labelling his team's defence soft after watching the Stormers run in six tries to one.

"We had four backs up all night vomiting - Cummins, Mafi, Dellit and Fairbanks were on drips all night," Graham said.

"But at the end of the day that wasn't the reason why we didn't win. We dropped some re-starts cold and things like that, and that's got nothing to do with a virus.

"I think we gave possession to them too many times in too many good places.

"You drop two kick-offs cold and they score two tries from possession they haven't really earned.

"Those sort of things hurt you.

"Our tackle miss was low but we were passive in the tackle.

"I thought our defence, which has probably been our most pleasing aspect of our season, was soft today."

The result marred what should have been a memorable night for Force skipper Nathan Sharpe, who became Super Rugby's most capped player after surpassing George Gregan's tally of 136 appearances.

"By far that's our worst performance of the year," a disappointed Sharpe said after the match.

The Force trailed 27-16 at half-time but failed to notch a point thereafter as the Stormers roared to their fifth win on the trot, lifting the South African side to top of the table.

Stormers centre Jacque Fourie and hooker Deon Fourie crossed for doubles, with Jean de Villiers and Andries Bekker adding the others.

James O'Connor was the rare shining light for the Force, posting all of his side's points through one try which he converted plus three penalties.

But his Stormers counterpart Peter Grant was even better, nailing all nine of his shots on goal to finish with a 21-point haul.

It was a sublime performance from last year's beaten finalists, who for the first time this season showcased their attacking flair after entering the match with just two tries to show from four outings.

"We didn't have any pride in possession whatsoever," Graham said.

"I think we held onto the ball once from a period of phases that were constructive and we scored a try.

"Everything else - we tried the miracle ball and turned the ball over early in the phase or we just lost our way.

"You're never going to build any pressure on the best defensive side doing that."

The Stormers secured the four-try bonus point just 45 seconds into the second half, with hooker Deon Fourie leaving the Force defence red-faced after regathering his own chip in a magnificent 50m run to the line.

The result left the Force with just one win and a draw from five games ahead of Saturday's crunch home clash with the Melbourne Rebels.

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