Schalk Burger scored the only try as the Stormers climbed to the top of the Super 15 table with a tight 16-6 victory over the Sharks in Durban.
The only touchdown came early in the first half, with the visiting captain capitalising on good work by his South Africa team-mates Jaque Fourie and Bryan Habana.
Peter Grant kicked the remaining 11 points to make it six wins from six for last year's finalists.
Meanwhile, the Reds recovered from a poor start to make it five wins from six thanks to a battling 30-25 victory over the Lions in Johannesburg.
The Australians played half of the opening 40 minutes with 14 men after yellow cards to James Slipper and captain James Horwill.
They had fallen 11-0 behind thanks to a try from Michael Killian and the boot of the inconsistent Andre Pretorius, but scores from Luke Morahan, Quade Cooper and Digby Ioane saw them to victory.
Jaco Taute scored the Lions' only other try, but Mike Harris' 15 points proved decisive at Ellis Park.
Melbourne Rebels kept their slim finals hopes alive with a thrilling 26-25 victory over the Western Force in Perth, but the Hurricanes are facing a real battle to save their season after slipping to a 26-14 defeat against the Bulls in Napier.
The combative Cheetahs had no match for the core experience provided for the Blues by front-row internationals Keven Mealamu and John Afoa as the Auckland side edged a scrappy match 29-22 at Toll Stadium in Whangarei.