Watford 0 Chelsea 1: Substitute Salomon Kalou scored in injury time to keep Chelsea's Premiership hopes alive as they moved back to within six points of leaders Manchester United.
Just when it appeared Jose Mourinho's team would be held to a goalless draw by bottom side Watford, Kalou flicked Andriy Shevchenko's cross beyond Ben Foster and sparked huge celebrations among the Chelsea players and staff.
The result was harsh on Watford, who matched the champions and saw Damien Francis and Steve Kabba miss glaring chances, but Chelsea once more displayed their enduring ability to claim victory while playing well below their best.
Foster's parent club Manchester United would have been hoping for an inspired performance from the recently-capped England goalkeeper, but Chelsea were struggling to test him.
In the 27th minute, Foster had plenty of time to gather Shaun Wright-Phillips' low shot from 20 yards, while a minute later Didier Drogba sent an angled effort high and wide to the delight of the home supporters behind Foster's goal.
But Foster did show his quality in the 33rd minute, producing an excellent instinctive stop with his leg to deny Drogba from eight yards after Lampard's header had found the Ivorian unmarked inside the penalty area.
Shortly afterwards, Michael Ballack should have done better than head Carvalho's left-wing cross wide of the left post.
Foster displayed his talent once more in the 44th minute, first blocking Ashley Cole's cut-back with his foot, and then pushing away Shevchenko's follow-up effort while under pressure from Drogba.
Knowing his team had under-performed in the first period, Mourinho sent them out early for the second half and also made two substitutions, Paulo Ferreira and Kalou replacing Geremi and Makelele.
But the changes did not dampen Watford's resolve and Kabba missed a wonderful chance to give them the lead in the 50th minute.
After Dan Shittu had flicked the ball on and Henderson had worked the ball to his strike partner, Kabba found himself with a clear shooting opportunity six yards out but the former Sheffield United man panicked and missed the target.
Mourinho and Watford boss Aidy Boothroyd were involved in a frank exchange of views on the touchline in the 54th minute after Rennie awarded a Chelsea free-kick, rather than a Watford corner, when Smith and Ferreira had contested possession. The pair had to be separated although both were smiling as they walked back to their respective technical areas.
In the 88th minute Francis could have won it for Watford but shot straight at Cech from four yards following Jordan Stewart's corner.
But just when it seemed as though Chelsea would be denied the win, Shevchenko escaped on the right in injury time and crossed for Kalou to head past Foster. PA
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