Former Arsenal striker Olivier Giroud and Willian on target as Chelsea smashed Watford 3-0 in the Premier League encounter at Stamford Bridge on Saturday night.
Manchester United had leapfrogged the Blues – who lost 3-2 at West Ham in midweek – into fourth place by thrashing Bournemouth earlier in the day.
However, Lampard’s side responded with a comfortable win over the struggling Hornets to reclaim the final automatic qualification spot for next season’s Champions League.
Olivier Giroud opened the scoring for the Blues, latching on to Ross Barkley’s clever pass and steering a left-foot shot into the bottom-right corner.
While Watford worked hard to contain the hosts, they offered little to suggest they were capable of a first win at Stamford Bridge since 1986.
And they fell further behind before the break, with Etienne Capoue’s rash challenge on Christian Pulisic resulting in a Chelsea penalty that Willian converted.
Thereafter it was relatively plain sailing for Lampard’s side, who rounded off the scoring when Barkley found the top left corner from Cesar Azpilicueta’s cross.
The Blues have now won three of their four games since the top flight resumed in June.
Nigel Pearson’s Watford remain a point above the relegation zone but have played a game more than 18th-placed Aston Villa, who travel to champions Liverpool on Sunday.
Chelsea travel to Crystal Palace in their next Premier League outing on Tuesday, 7, while Watford host bottom club Norwich on the same date at the same time.