With the tackles flying in, it was no surprise that a mis-timed challenge led to West Ham’s 23rd minute opener.
The ball fell for Hammers subtitute Luis Jimenez, who looked to be tripped by Hayden Mullins but replays showed that Michael Brown was the culprit and Diamanti fired in from the spot to the keeper’s left.
In the second-half Pompey sub Kevin-Prince Boateng almost embarrassed Herita Ilunga when the full-back left a ball for Robert Green to collect. Boateng took the ball away from the goalkeeper and defender but his finish went wide.
Pompey striker Frederic Piquionne had an effort saved late on before Kovac headed home Jimenez’s free-kick a minute from full-time to seal the win.