The Pretoria-based Bulls jumped to a five-point lead after seven rounds in the southern hemisphere tournament with a 28-15 victory over the winless Force in Perth on Saturday.
It was the first match of the Bulls' four-match Australasian tour and the defending champions were made to fight all the way against the Force.
Fellow South Africans Western Stormers and seven-time Super rugby champions Canterbury Crusaders had weekend byes and hold on to their second and third positions in the standings with seven rounds left to the play-offs.
The Brumbies consigned last season's finalists Waikato Chiefs to their third straight defeat with a 30-23 win at home on Friday, while the Reds registered a convincing 31-10 win over the Central Cheetahs in Bloemfontein.
New Zealand's Wellington Hurricanes slumped to eighth spot after their fourth straight defeat, this time going down 29-26 to the Coastal Sharks at home after a 50-metre penalty by Ruan Pienaar on full-time.