Pompey saw off Cambridge to maintain their six-point lead at the top of the League One table.
But they had to battle from behind before eventually securing a valuable 3-1 victory at Fratton Park.
John Mousinho's men fell behind when a Danny Andrew header was adjudged to have crossed the line.
Kusini won and converted a penalty to level the contest, however, before Myles Peart-Harris and Abu Kamara completed the turnaround.
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