Harry Redknapp fielded a strong team, making only three changes from Saturday's Wembley victory over West Bromwich Albion.
West Ham shaded the first half but they failed to take their chances. Pompey's five-man midfield gradually took control and, not for the first time, Redknapp proved that he had too much know-how for the club he both played for and managed.
Even the introduction of 18-year-old super kid Freddie Sears failed to spark the Hammers into life in the second period.
West Ham had the better of the early exchanges with Luis Boa Morte testing David James. The England keeper also had to react quickly to head the ball clear from the on-rushing Dean Ashton on the edge of his penalty area.
The Hammers ought to have taken the lead in the 17th minute when Bobby Zamora found himself free and on side inside the box, but having done the difficult part in side-stepping James, the striker screwed the ball across the face of the goal and wide of the far post.
Pompey responded well and went close themselves seven minutes later when David Nugent fired just wide.
Portsmouth bossed much of the second quarter but it was the Hammers who came closest to taking a half-time lead. Firstly, Ashton warmed James' hands with a fierce 35-yard drive and then Nolberto Solano saw his goalbound free-kick deflected wide of the goal frame.
The second half started at an even sleepier pace than the first until Niko Kranjcar livened matters up with a fierce drive that flew just over the bar.
It was the skilful Kranjcar who broke the deadlock in the 62nd minute with a crisp snap shot from the edge of the area.
Portsmouth controlled much of the remainder of the proceedings with West Ham's huff and puff producing no end product.
Although young Sears darted around the box, it was actually another West Ham substitute, Carlton Cole, who injected the most energy into a fading home side.
The match ended with Portsmouth still in control and the home fans went home disappointed.