Has supporting Portsmouth ever been so much fun? Certainly not since 1950, when the South Coast club retained the title in front of more than 50,000 fans at Fratton Park. The capacity of their ground may be only 20,000 these days, but no one is complaining. Portsmouth have money to burn, they are going to Wembley and Southampton, their fierce local rivals, are going nowhere.

If that is not enough, Portsmouth also have David James, and the 37-year-old England goalkeeper showed his enduring quality last night by producing a world-class save during the second half of a pulsating encounter that ended with Portsmouth climbing two places to seventh in the table.

Harry Redknapp’s team won, but they did it the hard way, squandering a two-goal lead before Hermann Hreidarsson made it 3-2 and Kanu wrapped up the victory with a close-range header. “I thought it would be difficult, but we made a great start,” Redknapp said. “It is nice to bang in four and credit to Birmingham for coming here to play.”

Redknapp made three changes to the team who qualified for the semifinals of the FA Cup by beating Manchester United at Old Trafford on Saturday. The manager’s safety-first approach was replaced by a conventional 4-4-2 formation, with Jermain Defoe and Milan Baros replacing Kanu up front and Pedro Mendes taking the place of John Utaka in midfield.

At Old Trafford, Portsmouth defended like lions and rode their luck to knock out United. Last night, they attacked as if their lives depended on it from the first minute. Defoe made the early breakthrough from the penalty spot in the sixth minute after David Murphy was judged by Steve Tanner, the referee, to have blocked a cross by Baros with his left hand. Defoe scored again three minutes later — his fifth goal in five matches — when Maik Taylor could not hold on to stinging strikes by Sulley Muntari and Baros.

So far, so good for James, the England goalkeeper who started the day by extending his contract at the club until 2010. With Portsmouth 2-0 up after only nine minutes, James was probably looking forward to an easy night, but he was beaten a minute later when Liam Ridgewell flicked on a corner taken by Sebastian Larsson and Fabrice Muamba slid in at the far post to bring Birmingham City back into the match.

Desperate for points to climb away from near the relegation zone, Birmingham regrouped and equalised on the stroke of half-time through Larsson. The former Arsenal midfield player scored with a stunning free kick from 25 yards that rose and dipped through Portsmouth’s disintegrating wall as James dived in vain.

Portsmouth should have been ahead at half-time, but Fratton Park regulars did not have to wait long for their team to regain the lead. Ridgewell and Radhi Jaidi were having trouble with set-pieces taken by Muntari and when the Ghana midfield player floated in a free kick after 48 minutes, Ridgewell could only deflect the ball into Hreidarsson’s path and the Iceland defender made no mistake from close range.

Alex McLeish, the Birmingham manager, responded by sending on James McFadden, who was on the bench because his wife had given birth at 1am yesterday, and although the Scotland forward’s touches added an edge to Birmingham’s attack, Portsmouth always looked more dangerous. Baros, dribbling with his head down into cul-de-sacs, can be infuriating to watch, but the Czech Republic forward was having one of his more effective games and he received a standing ovation when he was replaced by Kanu in the 71st minute.

Kanu was sent on to help his team to retain possession shortly before James made a stunning point-blank save to stop Cameron Jerome making it 3-3. The Nigeria forward gave Birmingham’s tiring legs another problem to deal with and he wrapped up the victory by heading in a cross from Mendes near the end.

“I can’t fault the commitment and spirit of my players,” McLeish said as his team remained in seventeenth place. “We went for it, but got punished. We have a huge game against Newcastle United on Monday.”

Portsmouth (4-4-2): D James – G Johnson, S Campbell, S Distin, H Hreidarsson – P Bouba Diop (sub: L Diarra, 61min), P Mendes, S Muntari, N Kranjcar – J Defoe (sub: Lauren, 85), M Baros (sub: Kanu, 71). Substitutes not used: J Ashdown, J Utaka.

Birmingham City (4-4-2): M Taylor – S Kelly, R Jaidi, L Ridgewell, D Murphy – S Larsson, F Muamba, D Johnson, G McSheffrey – M Zárate (sub: J McFadden, 61), M Forssell (sub: C Jerome, 71). Substitutes not used: C Doyle, M Nafti, S Parnaby. Booked: Zárate.

Referee: S Tanner.