Portsmouth brought Middlesbrough back down to earth with a stunning display in a 4-0 win at the Riverside Stadium, which maintains their unbeaten start to the season.

Five days after the Teessiders had humbled champions Chelsea to give manager Gareth Southgate his maiden Barclays Premiership victory, Boro were torn apart by African strike-force Kanu and Benjani Mwaruwari.

Kanu fired the visitors into a seventh-minute lead and then killed the game off 12 minutes after the break after running from his own half to score his fourth goal of the season.

In the meantime, Mwaruwari had added a 50th-minute second with a far post header to leave Boro in disarray, and substitute Svetoslav Todorov simply rubbed salt into the wounds with a fourth in injury time.

It was a salutary lesson both to Southgate and new Sunderland boss Roy Keane, watching from the stands among a crowd of just 24,834 hours after signing a three-year contract at the Stadium of Light, of just how quickly fortunes can change in football.

In the wake of Wednesday night's famous victory over the champions, Southgate had borrowed a chapter from the Steve McClaren book of football coaching.

Consistency, he said, was the key for a side which last season beat the Blues, Arsenal and Manchester United at home, yet still finished 14th in the league table.

But just as they did against Jose Mourinho's side, they found themselves trailing to an early goal, and this time it came anything but against the run of play.

Indeed, had keeper Mark Schwarzer, playing the 300th league game of his career on Teesside, not been at his best with just six minutes gone, the visitors would have been in front a minute earlier.

Chris Riggott was penalised for a foul on Kanu, and Pompey skipper Dejan Stefanovic stepped up to curl a free-kick towards the top corner, where the Australian needed every inch of his sizeable frame to claw the ball away.

However, his efforts counted for little when Portsmouth took the lead from the resulting corner.

Mwaruwari was allowed to help David Thompson's cross back across goal, where Kanu thumped it past Schwarzer with the minimum of fuss.

Boro responded swiftly with Stewart Downing forcing a solid save from former England keeper David James and Linvoy Primus just preventing Mark Viduka from reaching Lee Cattermole's header at the far post.

But the visitors were proving a threat on a more regular basis as Thompson tested Schwarzer with a long-range free-kick and Stefanovic headed over from a Sean Davis free-kick.

Boro belatedly found their rhythm and former Pompey striker Aiyegbeni Yakubu whistled in a 33rd-minute shot which James had to smother before Gaizka Mendieta drilled a 40-yard free-kick just wide seven minutes later.

But, in the meantime, the lively Kanu had forced an excellent save from Schwarzer with a sweetly-struck drive on the turn, and the visitors were good value for their half-time lead.

Southgate's side returned determined to right the wrongs of the first half, and Viduka went close when he met Stewart Downing's 47th-minute free-kick at the far post with a firm header, although he could not keep his effort down.

Yakubu wastefully scuffed a long-range effort wide after Gary O'Neil had gifted him possession, but worse was to follow with 50 minutes gone.

Boro were still trying to organise for a defensive corner when the ball was played short to Thompson, who lifted a cross to the far post.

There were blue shirts queueing up, but it was Mwaruwari who climbed highest to power a downward header into the net.

The Teessiders' response was to pepper James' goal, Mendieta whistling a volley just over, Downing seeing a well-struck free-kick tipped away and Riggott forcing another smart save from James.

But with Boro sensing the start of a fightback, disaster struck when they were felled by a sucker punch.

Pompey were defending desperately and the ball was hacked up towards Kanu inside his own half, and even when he turned Emanuel Pogatetz, there was plenty still to do.

But the Nigerian raced away from the cover and, after holding off the chasing George Boateng, lifted a shot over the advancing Schwarzer to make it 3-0.

Lee Cattermole kept James on his toes with a rising 68th-minute drive from distance and Viduka was more wayward from similar range two minutes later, and it was then that Southgate made his last desperate throw of the dice.

Viduka, Mendieta and Cattermole made way for Malcolm Christie, Fabio Rochemback and James Morrison, but Todorov completed the rout in injury time when he lifted a Davis pass over the helpless Schwarzer with the home defence having dissolved.