Wayne Rooney climbed off the bench to produce a virtuoso performance as Manchester United overcame plucky Portsmouth 2-1 at Old Trafford.
The striker, who had ended his goal drought against Arsenal last week, came on to finish a well-worked move on 77 minutes and six minutes later chipped what proved to be a majestic winner.
Kanu deflected Pedro Mendes' shot past Tomasz Kuszczak with three minutes left, but United saw out the game to move into the fifth round of the FA Cup.
A feisty first half saw controversy dominate the opening 15 minutes, with both teams sensing reason to be aggrieved with the officials.
Pompey could have had a penalty as early as the fourth minute when Gary O'Neil skipped into the box to latch on to former United striker Andy Cole's flick-on.
O'Neil got to the ball before Kuszczak, but the United keeper was able to get a glove on the ball as the Pompey midfielder sniffed an opening goal.
The visitors were sprightly in the early stages, with Richard Hughes shooting over and Matthew Taylor having a low effort saved.
On 13 minutes, it was the home side's turn to be upset with the officials when Nemanja Vidic's header was hacked from behind the line by Pedro Mendes after David James had made a superb one-handed save.
The irony was that Mendes was denied a perfectly valid goal at the same end two years ago when the then-Tottenham midfielder's long-range shot was clawed back from behind the line by Roy Carroll.
United then held the upper hand for the remainder of the half, but their attacks regularly foundered on the rock at the heart of Portsmouth's defence - the impregnable Sol Campbell.
Park Ji-sung failed to connect to Patrice Evra's fizzing cross, while James hurriedly cleared the ball away after Lauren inadvertently deflected a Ryan Giggs cross which had been pulled too far behind Henrik Larsson.
From the resultant corner, James produced an alert one-handed stop to turn over Michael Carrick's header from Giggs' centre.
The start of the second half again saw Pompey look lively and Linvoy Primus glanced a Taylor free-kick on to the roof of the net.
Cole was then presented with a great chance to score against his old employers when teed up by Mendes, but his tame shot was blocked by Rio Ferdinand.
United responded with gusto and had the ball in the net on 56 minutes when Larsson finished emphatically, but he was adjudged to have made his run a fraction too soon, despite TV replays suggesting the contrary.
Further pressure saw James fling himself across goal to turn away a Paul Scholes howitzer and the keeper then made a more routine save to claim Larsson's awkward attempt from Carrick's deflected cross.
Pompey were keeping the Premiership leaders at arm's length, but succumbed to a slick move 13 minutes from time when Larsson released Giggs and his low cross was converted by Rooney.
Benjani Mwaruwari and Rodolph Douala were brought on to try and fashion an equaliser, but no sooner had Pompey adopted a more positive outlook than they had slipped further behind.
Rooney collected Gary Neville's pass 30 yards from goal, took a couple of strides forward and clipped a sumptuous chip over James, who could only stop and stare in admiration.
Yet Pompey's fight was far from extinguished and they rallied three minutes from time when Mendes was set up by Kanu and his low drive deflected off the Nigerian and past a wrong-footed Kuszczak.
Park smashed a volley against the post towards the end, and Rooney could have had a hat-trick, but lashed wide after impudently nipping past Campbell, who had produced a vintage display in the previous 90 minutes.