Pompey's dreadlocked star headed two first-half goals to give Harry Redknapp another happy return to Upton Park. Teddy Sheringham's late effort for the Hammers was simply not enough to prevent Alan Curbishley's side from crashing to a dismal defeat.
Following the Hammers' vital victory over Manchester United and gritty goalless draw at Fulham, the whole of the East End came into this festive fixture hoping that new boss Curbs could make it a hat-trick of unbeaten games for the first time this season.
Languishing precariously in 18th place, West Ham made two changes from the side that earned a point at Craven Cottage on Saturday, as Yossi Benayoun and fit-again Danny Gabbidon made a welcome return in place of Lee Bowyer and Christian Dailly.
Back in March, Redknapp returned to Upton Park for the first time since leaving the club in 2001, and duly plundered a 4-2 win for relegation-threatened Pompey who looked all set for the Championship.
Now, ten months on, he headed through the Green Street gates with his new-look, South Coast side in the heady, heady heights of sixth spot and looking firmly across the English Channel towards Europe.
Following the weekend win over Sheffield United, Redknapp made just one enforced switch as hamstring victim Benjani Mwaruwari made way for Manuel Fernandes but, with Kanu single-handedly teasing and tormenting the Hammers up front, Pompey hardly missed their £4.1million Zimbabwean striker as they quickly gained the initiative in the opening stages.
Indeed, only 16 minutes were on the clock when Pedro Mendes floated the visitors' first corner of the day to the far post, where Primus out-jumped the static Marlon Harewood to head home his first goal of the season from six yards.
And things could have turned even worse for Curbishley's strugglers midway through the half when the Pompey scorer nodded Matt Taylor's corner across goal and Sol Campbell lashed high over from ten yards after Sean Davis' overhead kick had been blocked on the line by Hayden Mullins.
The five-man Portsmouth midfield was ambushing the outnumbered Hammers quartet at will and, eight minutes before the break, it was no surprise when Pompey grabbed their second goal with a carbon copy of their opener.
This time, Jonathan Spector's handball allowed Mendes to deliver an inch-perfect free-kick to the far post and, this time, Primus stole above Robert Green and Mullins to send his second header of the day crashing into the home net.
The sum total of the lack-lustre and lackadaisical Hammers' first-half efforts had amounted to a couple of Benayoun efforts and a Bobby Zamora sizzler that stung the hands of the otherwise redundant former Upton Park keeper David James.
Quite simply, something had to be done and Curbishley responded by unveiling Pompey old-boy Sheringham and Carlos Tevez in place of the desperately disappointing duo of Harewood and Matthew Etherington for the restart.
That brought the biggest cheer of the afternoon from the home fans amongst the 34,913 crowd but, with West Ham having only managed two goals in a game twice this season, it always looked a big ask from the new look Anglo-Argentinian attack.
However, Sheringham soon forced James into a comfortable low save, while Benayoun started to instil some vibrancy and the tricky Tevez also raised the tempo with a couple of sizzling shots that just missed the target.
With ten minutes left, West Ham finally gave themselves a glimmer of hope when Anton Ferdinand ruffled the hitherto unflustered Campbell eight yards out and, in the confusion, Sheringham coolly clipped an angled ball beyond the stranded James towards the far post, where ex-Hammer Glen Johnson could only help the ball over the line.
The predictable late cavalry charge ensued but, in truth, West Ham's woeful first-half showing had caused irreparable damage and, for the second season running, it was Redknapp who deservedly returned to the South Coast with all three points, while Curbishley was left to reflect just how he's going to get his confidence-sapped Hammers out of that bottom three.