The South African side had Renaldo Bothma sent off in the 41st minute, but remained the superior force, adding a further two tries to inflict a fourth straight defeat on Sunwolves.
Although Hayden Cripps missed the conversion, a red card for Bothma for a high tackle on Ed Quirk looked likely to see momentum remain with the Japanese side.
However, the hosts built on their lead when Jason Jenkins powered over and Serfontein touched down his second of the match in the 65th minute.
A try from Quirk denied the hosts a bonus point, while Liaki Moli thought he had given Sunwolves a chance for a losing bonus point when he crossed in the final minute of regulation time.
The conversion was sent wide, but the referee – who had lost his communications ear piece earlier in the game – was told by an assistant before the kick to review the decision, and a knock-on was belatedly given.
#BULvSUN FT! Vodacom Bulls 34-21 Sunwolves.
— Official Blue Bulls (@BlueBullsRugby) March 17, 2017
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