Flyhalf Marnitz Boshoff booted 22 points, including two drop-goals, to dampen the playoff chances of the Bulls as the Lions completed a surprise 32-21 Super Rugby win at Ellis Park.
The defeat means the Sharks will win the South African conference and leaves the Bulls to scrap for a wildcard place.
They currently sit ninth and trail the Wellington Hurricanes in sixth position by four points with two rounds remaining.
Boshoff had the Lions in front from start to finish as he also kicked two conversions and four penalties to go with tries from centre Alwyn Hollenbach and flanker Warwick Tecklenburg.
It took the Bulls 70 minutes to cross the try line as centre William Small-Smith cantered over unchallenged, followed shortly afterwards by flanker Jacques du Plessis, while Jacques-Louis Potgieter kicked three penalties and a conversion to complete their scoring.
