Birmingham consolidated their promotion challenge as they impressively chalked up a seventh successive home win over relegation-threatened Portsmouth.
Pompey, fresh from their 1-0 win at Ipswich last Saturday, battled bravely but were eventually defeated by a smartly-taken goal from Birmingham substitute Marcelo.
Marcelo's introduction at half time was an inspired move on the part of Blues boss Trevor Francis.
Following a disappointing goalless first half Francis obviously decided that there was a need for a more determined attacking formation.
He drafted Marcelo into the attack in place of Stan Lazaridis - a move which immediately paid dividends.
Within four minutes of the restart Birmingham skipper, Martin O'Connor, won possession from former colleague Steve Claridge and raced forward before playing an accurate pass into the path of Marcelo.
The Brazilian, who joined Birmingham from Sheffield United earlier in the season for £350,000, cut inside Portsmouth skipper Adrian Whitbread and scored with a low shot past Russell Hoult.
Portsmouth had a lucky let-off a minute later when Whitbread clattered Marcelo to the ground in the penalty area - but no penalty was awarded.
The referee was often in a lenient mood as there were other suspect tackles which warranted the yellow card.
Portsmouth included new signing Kevin Harper, but he wasted a fine chance of a fairytale debut when he carelessly wasted Pompey's best goalscoring attempt just before the interval.
It proved to be an expensive mistake as Birmingham stepped up the tempo and immediately scored their vital winner through Marcelo to make them the division's most in-form team.