Of all the contenders at the top of the table, Sir Alex Ferguson will be worried most about Chelsea after they showed the quality none of the other teams possess last night — the ability to win ugly when it really counts.

With seven 1-0 victories of their own this season, Manchester United have hardly been the great entertainers but over the past five years Chelsea have shown they can grind out wins without worrying about the beautiful game like no one else.

When Ferguson's side thrashed Chelsea 3-0 less than three weeks ago, the Blues looked in disarray under beleaguered manager Luiz Felipe Scolari, however it was not the knockout blow many expected.

The style of their performances may not have improved since then, yet the refusal to give in, which was instilled in them by former manager Jose Mourinho, is crucially alive and well.

Ferguson's men know they were made to fight until the end by Chelsea in the past two seasons and that things could easily have gone the other way.

Scolari may not have the strength of squad that Ferguson has, but his players are used to the pressures of the title run-in, unlike the other challengers to United's crown in the top five.

Liverpool, who face Chelsea on Sunday, are showing the strain having drawn seven of their last 10 games, Arsenal continue to be inconsistent and their point at Everton now makes them even stronger outsiders while Aston Villa are still an unknown quantity.

When Chelsea had to travel to Southend in an FA Cup third-round replay just three days after being humiliated by United, it seemed like an unwelcome distraction. But the club's 4-1 victory appeared to provide a quick antidote to the damage inflicted at Old Trafford and they have secured three more wins since. Captain John Terry certainly believes the Cup gave them an opportunity to rediscover their form.

Writing in his programme notes last night, he said: “The FA Cup has been good to us this year because it's given us the chance to string together a good run of results.

“We hadn't won three games in a row for a while but with the replay at Southend, the late show against Stoke [in the League] and the win on Saturday against Ipswich, we're building back our confidence.”

It is clearly still a work in progress as they laboured for long periods last night against a Middlesbrough side who look serious candidates for relegation.

Very few clear-cut chances were created from open play in the 2-0 win, although things could have been made easier for Chelsea if Ashley Cole hadn't headed Salomon Kalou's cross into the side netting from just a few yards out in the 13th minute.

The introduction of Didier Drogba as a second-half substitute did improve Chelsea offensively, even though Scolari decided once again to move Nicolas Anelka to the wing to accommodate him rather than play the two strikers together down the middle.

If the Brazilian wants to get the win he needs and Ferguson fears at Liverpool on Sunday, then he should start the game with both, even if he keeps the 4-3-3 formation.

For while Drogba isn't quite up to top form yet, his power flanked by the pace of Anelka and Kalou poses so much more of a threat.

Kalou, who scored both goals against Middlesbrough, certainly feels they work well as a trio and said: “We all have pace and that creates space.

“I scored two goals because they made a lot of space for me and I will do the same for them in the next game.

“We are working well together and that is good for the team. It's very important to keep Didier here and have him in the team because you know he has the ability to score goals and scare defenders. That's why he's there.”

Middlesbrough certainly began to look ragged the longer the second half went on and conceded more and more free-kicks.

Chelsea's problems at the back of late have been because of set-pieces and yet they were the ones to benefit this time. Gareth Southgate's side failed to deal with a corner from Frank Lampard to their cost on two occasions.

Kalou made the most of a poor attempt to head clear by David Wheater to volley home in the 58th minute and the striker was then completely unmarked to score the second nine minutes from time.