Paddy Kenny suffered the cruellest of fates as Sheffield United's FA Cup dream came to an agonising end at Middlesbrough.

The goalkeeper had repelled everything the Barclays Premier League side could throw at him when, 23 minutes into extra time, in his desperation to claim Mido's deflected shot, he helped the ball into his own net to hand Boro a quarter-final clash with Cardiff.

It was the most unkind of misfortunes for the 29-year-old, who had turned in a superb display to keep out the Teessiders' record signing Afonso Alves - on two occasions - Luke Young, Stewart Downing and Tuncay Sanli.

Indeed, had James Beattie's 90th-minute shot crept inside Mark Schwarzer's left post rather than gone marginally outside it, it would be the Blades welcoming Cardiff to Bramall Lane in the last eight.

On the balance of play, Boro were good value for their victory, but having dominated for long periods, they had failed to make the most of their pressure until they were handed a large slice of good fortune in front of a crowd of 28,108.

With Gareth Southgate's side needing replays in each round of last season's competition before going out in the quarter-finals at the second attempt at Manchester, and the Blades having drawn their last six matches, the chances of the tie ending after 90 minutes always looked remote.

However, had Alves had his way, Boro could have been all but home and dry by the time the whistle brought an end to the first half.

The Brazilian could barely believe his luck when nine minutes into his full debut, he ran unmarked onto Fabio Rochemback's corner in front of goal.

A square connection would have seen the ball fly into the net with Kenny stranded, but his glancing touch simply sent it wide of the far post and let the visitors off the hook.

But a man who scored 49 goals in as many games for previous club Heerenveen was never likely to dwell on that miss or to have to wait long for another opportunity, and his chance duly arrived 10 minutes before the break.

Matthew Kilgallon was booked for his trip on Alves 30 yards out, but it was Kenny who had to pick up the pieces and the South American blasted in a dipping, swerving free-kick which the keeper did well to parry.

Downing had earlier gone close to celebrating his signing of a new five-year contract in style when he exchanged passes with Rochemback and then dragged a right-foot shot just across the face of goal.

However, it was in the unlikely shape of full-back Young that Boro threatened as the half drew to a close, the defender seeing a stabbed close-range effort turned away by Kenny's foot before shooting into the side-netting in injury time.

If the visitors had to defend for dear life at one end, they were only sporadically as busy at the other with Beattie making his presence felt at times, but to no great effect.

However, he might have done better with an 18th-minute volley, but scuffed his shot straight at Mark Schwarzer.

Central defenders Chris Morgan and Kilgallon found themselves in the wars as the second half got under way, both receiving accidental blows to the head.

However, skipper Morgan had recovered his senses in time to make a vital block by the time Alves cut inside from the left and went for goal on 50 minutes.

But as the pressure mounted on the Blades, Derek Geary followed Kilgallon and Morgan into referee Peter Walton's notebook for a late challenge on Downing to leave manager Kevin Blackwell with a real headache.

Schwarzer had to clear Emanuel Pogatetz's back-pass hurriedly as Billy Sharp chased it down, but was otherwise largely a spectator, and Blackwell turned to substitute Lee Martin for inspiration.

However, Kenny had to come to the rescue once again on 65 minutes, clinging on to Alves' driven shot under extreme pressure after Julio Arca had broken from his own half to feed the striker.

Alves' evening came to an end on 73 minutes as he made way for Tuncay, but the visitors sensed their opportunity and mounted a strong finish with Beattie again prominent.

The former Blackburn striker fired agonisingly wide from Gary Speed's low cross in the final minutes of normal time, but the tie was destined to go another half-hour.

Kenny once again kept his side in it with a fine double save from Downing and Tuncay seven minutes into extra time, but it was he who ultimately handed Boro their place in the last eight through a cruel twist of fate.

Mido's shot reared up off the prone Morgan and looped up on to the post, and as it dropped towards goal, the keeper could only palm the ball into his own net.

Teams

Middlesbrough Schwarzer, Young, Hines, Pogatetz, Grounds,Boateng, Arca (O'Neil 77), Rochemback (Johnson 104), Downing,Alves (Sanli 73), Mido.

Subs Not Used: Turnbull, Cattermole.

Booked: Hines, Pogatetz, Downing.

Goals: Kenny 114 og.

Sheff Utd Kenny, Geary, Morgan, Kilgallon, Naysmith,Stead (Martin 59), Quinn, Speed, Tonge, Beattie,Sharp (Hulse 73), Martin (Shelton 91).

Subs Not Used: Armstrong, Ehiogu.

Booked: Naysmith, Morgan, Geary, Quinn.

Att: 28,108

Ref: Peter Walton (Northamptonshire).