Hermann Hreidarsson heads home in an all-action goal-fest at Fratton Park.
Pompey beat Reading 7-4 on September 29, 2007, which remains the highest-scoring match of all time in the Premier League.
Two Benjani strikes had the Blues cruising before Stephen Hunt struck on the stroke of half-time and David James then gifted Dave Kitson an equaliser.
But Hreidarsson restored the lead before James atoned for his error by keeping out a Nicky Shorey penalty.
Benjani restored the two-goal lead to claim his hatr-trick and Niko Kranjcar, Sean Davis and Sulley Muntari sealed victory, with Shane Long and Shorey responding late on.