Until Salomon Kalou headed in Andriy Shevchenko's cross in the third minute of injury time, it looked like the title race was all over following Manchester United's 4-1 win over Blackburn earlier in the day.
It would be wrong of Jose Mourinho's men to claim they deserved the victory, but it showed exactly why they are the champions and will not give up on their trophy until it is mathematically impossible.
The outcome was rough on Championship-bound Watford, who pushed the millionaires of Kings Road all the way with the type of performance that has not been seen often enough and underlines why their fate is now all but confirmed.
Hornets boss Adrian Boothroyd cleverly opted to go like-for-like with Chelsea in the centre of the park and, if the visitors thought they were in for an easy ride, their illusions were shattered within the opening minutes when Darius Henderson pole-axed John Terry from a corner.
Watford continued to force the early pace with Tommy Smith having a strong penalty appeal turned down, Steven Kabba forcing Petr Cech into an acrobatic save and Henderson fizzing a shot on the turn narrowly over.
Chelsea though soon began to find their attacking feet and it took an excellent stop from Ben Foster to prevent Didier Drogba netting from close range, while Michael Ballack also headed another good opportunity wide.
Watford refused to lie down after the break and Kabba missed another good opportunity.
But as the half wore on, the champions increasingly bought their class to the fore without ever looking at their fluent best.
It took some excellent defending from Jordan Stewart and Jay DeMerit to deny Shevchenko and Frank Lampard respectively, but as a draw looked increasingly inevitable, Watford almost snatched a famous win when Cech thwarted Damien Francis from a late corner.
But sadly for the battling Hornets, it was Chelsea who snatched the points to keep their title hopes alive.