HERNAN CRESPO inflicted an ultimately cruel defeat on Middlesbrough and repaid an instalment on his £16.8m fee as Chelsea narrowed the gap on Premiership leaders Arsenal to a point with a game in hand.

The Riverside was treated to a hugely-entertaining contest which witnessed a stirring second-half fightback from a Boro side who were distinctly inferior in the opening period.

Boro's three-match winning run should have been extended as they spurned a string of second-half chances after hauling themselves back into the game through Szilard Nemeth immediately after the interval.

Icelandic striker Eidur Gudjohnsen gave Chelsea a 17th-minute lead when George Boateng was robbed by Frank Lampard.

Damien Duff, the architect of both Chelsea goals, took two touches to deliver an incisive ball behind the defence to Gudjohnsen, who beat goalkeeper Mark Schwarzer at his near post with a neat right-foot finish.

Boro let themselves be exposed on the flanks to Chelsea's expansive game, which the visitors exploited to telling effect in the first 45 minutes.

But it was an infinitely more assertive Boro after the break, with Spanish star Gaizka Mendieta the galvanising force in midfield.

Inside 20 seconds of the restart, Boro were level when Mendieta's ball found Nemeth deep in the box and the Slovakian slipped his shot past keeper Carlo Cudicini for his third goal of the season.

But Boro allowed opportunities to go begging and were made to pay when Argentina striker Crespo, one of Claudio Ranieri's major recruits in the Roman Abramovich-inspired revolution at Stamford Bridge, struck two minutes from time to preserve his side's unbeaten League record.

Crespo, an interval replacement for the disappointing Jimmy Floyd Hasselbaink, escaped at the far post to execute a downward header on the end of Duff's curling cross from the right and grab his third goal since his August arrival from Inter Milan.

But Boro might have known it would end this way; they have now won only two of their last 17 games against Chelsea in all competitions since the Blues provided the opposition on the day the Riverside staged its first match eight years ago.

It's a very different Chelsea now, and interim chief executive Paul Smith revealed that Russian oil billionaire Abramovich is given a detailed run-down on tactics and team changes from manager Ranieri before every game.

There was certainly ample for him to assimilate on this occasion, with only four players surviving from the side that started the 2-0 home defeat to Turkish title holders Besiktas in the Champions League last week.

It meant there was literally an embarrassment of riches on the Chelsea bench - with over £45m worth of talent in the shape of Crespo, Adrian Mutu, Joe Cole and former Boro loanee Geremi, who was making his first return to the Riverside since his successful stint here last season.

But there scarcely appeared to be any need to call up the Bluechip Brigade as Chelsea set about Boro with the air of a side determined to get back in the business of winning.

With under a minute on the clock, the visitors came close to unlocking the Boro defence. Duff exchanged passes with Gudjohnsen before sliding the ball across goal. Gudjohnsen failed to meet it and when the ball reached Jesper Gronkjaer at the far post, his first-time shot was blocked by Schwarzer.

There was still no place in Boro's starting line-up for Juninho, even though manager Steve McClaren was without Bolo Zenden and knee ligament victim Joseph-Desire Job.

Dutch winger Zenden, on loan from Chelsea for the season, was refused permission to play by his parent club.

McClaren preferred Mendieta, back after a three-match absence with calf trouble, to Juninho while Nemeth returned in place of Job, who underwent surgery last week.

A couple of wayward drives from Brazilian Doriva was the best that Boro could muster in the opening half as Chelsea dominated.

Hasselbaink, restored to the Blues' starting line-up, blotted an otherwise exemplary first-half exhibition from his side with a crude, studs-raised lunge on Chris Riggott which prompted a furious reaction from Boro.

Colin Cooper squared up to Hasselbaink and Riggott pointed to the damage the Dutch striker had done to his leg before referee Mark Halsey flourished the yellow card.

Boro were complaining again minutes later when Danny Mills was harshly penalised for an aerial challenge on Cudicini.

But Chelsea almost doubled their lead in the 32nd minute when Lampard swung over a right-wing free-kick and England colleague John Terry emerged unmarked at the far post only to see his header come back off the bar.

German-born defender Robert Huth, 19, standing in for Marcel Desailly, was the second Chelsea player booked in the 36th minute for a reckless, high-footed challenge on Mills as Boro attempted to snuff out another attack.

But it was a rejuvenated Boro after the break and their renewed urgency unnerved a hitherto imperious Chelsea.

Mendieta's industry and invention were key to Boro's improvement and when he tackled back to beat Wayne Bridge in the 55th minute, his low centre just eluded Malcolm Christie - and the far post - with Cudicini at full stretch.

Both keepers then had to make superb reaction saves in quick succession.

First, Schwarzer did well to deal with Huth's swerving drive from a free-kick routine, and then Cudicini was almost similarly deceived by Doriva's wicked 20-yarder.

The Italian keeper became Public Enemy No 1 as far as the home fans were concerned when he feigned injury after a challenge in the box by Christie.

Much to the Boro striker's dismay, Mr Halsey raised the yellow card at him, but replays showed there was minimal contact as Christie made a legitimate attempt to win a 50-50 ball.

It was no surprise when Cudicini quickly sprang to his feet once Christie had been punished.

Cudicini's every touch was booed thereafter, and Christie should have made him squirm when he latched on to Mendieta's ball but lifted his finish over with only the keeper to beat.

Boateng then squandered another glorious chance for Boro.

The Dutchman tripped Lampard in the build-up but was allowed to go, and when Christie returned the ball to him on the right, Boateng cut inside before firing narrowly wide of the far post.

But Crespo gave Boro a warning they failed to heed when he caught Cooper in possession and unleashed a shot which would have flown into the net had it not hit Schwarzer full in the face. At the death, Boro thought they had a case for a penalty when substitute Stewart Downing went down under Glen Johnson's challenge, but Mr Halsey couldn't have been better placed and waved away the appeals.