Portsmouth's amazing home drought continued as Turkish striker Tuncay Sanli pounced on a David James fumble to hand Middlesbrough three points at Fratton Park.
It was Pompey's sixth consecutive scoreless home outing in the Premier League and the cheers from Wednesday's goalless draw with Arsenal soon faded away in a shot-shy display.
Papa Bouba Diop was outstanding for Pompey, running the show from midfield and filling the holes left by the injured Sean Davis, flu-victim Pedro Mendes and dropped Sulley Muntari.
But Diop could not convert his dominance into goals and, after Tuncay had struck for his fourth goal in six games in the 20th minute, fired over a golden chance to equalise just before half-time.
Diop's enormous figure was seemingly everywhere as he tried to rouse Pompey but the home fans probably wished it was anyone but him on the end of Benjani's cutback in first-half stoppage time. Diop has yet to score since his £3million move from Fulham in August.
Tuncay, however, was the first to show some purpose in a scruffy start to a frenetic encounter, shooting wide on the turn with James confidently looking on.
John Utaka's mazy run on the right eventually took him into the Boro area and, although he was challenged as he attempted to shoot, the ball ran loose for Niko Kranjcar to fire a low drive which Mark Schwarzer smartly held.
Sol Campbell had to time his tackle just right to deny Tuncay when Boro broke back and from the resultant corner Kanu was back defending, at the expense of another flag kick.
Neither side showed much creativity in the opening 20 minutes. Kranjcar had a goalbound free-kick blocked by an efficient Boro wall after Mohamed Shawky manhandled Kanu off the ball and Stewart Downing's set-piece was also blocked after a clumsy foul by Diop on Tuncay.
But then Boro gratefully collected a soft goal and a surprise lead as Pompey dithered at the back.
Tuncay started the move, seizing on the loose ball after Campbell and Lee Dong-Gook contested Luke Young's long punt forward.
He switched it right, from where Stewart Downing's cross sailed beyond the far post and O'Neil volleyed what looked like a shot of little power. James should have dealt with it but fumbled and Tuncay - despite looking suspiciously offside - followed up to knock into an empty net.
Tuncay wore an almost embarrassed smile as he ran to celebrate with former Pompey hero O'Neil, who had recovered from a hamstring injury to come in as one of four changes in the Boro line-up.
Strike partner Lee would have added another soon afterwards but for Hermann Hreidarsson's intervention but Pompey's reprisals were all stoutly resisted, even though battling Boro picked up four bookings in the second half when they pulled everybody back behind the ball.
The introduction of substitutes Matt Taylor and David Nugent gave Pompey an edge at last but when Richard Hughes made a rare attacking move from midfield and knocked down Taylor's free-kick in the 65th minute, Kanu shot against the post from six yards.
Pompey seemed destined not to score after that. Defeat was their first by Boro since 1994 and they have now gone 540 minutes without scoring at home.
TeamsPortsmouth James, Lauren, Campbell, Distin,Hreidarsson (Taylor 46), Utaka (Nugent 62), Diop,Hughes (Muntari 72), Kranjcar, Mwaruwari, Kanu.
Subs Not Used: Begovic, Traore.
Booked: Taylor.
Middlesbrough Schwarzer, Young, Woodgate, Huth, Pogatetz,O'Neil (Johnson 86), Shawky, Arca, Downing, Sanli,Lee (Hutchinson 75).
Subs Not Used: Turnbull, Riggott, Grounds.
Booked: O'Neil, Woodgate, Young, Arca.
Goals: Sanli 20.
Att: 20,089
Ref: Alan Wiley (Staffordshire).
