Watford 0-3 Southampton
Rickie Lambert scored a hat-trick to send Southampton back to the top of the Championship table with a comfortable victory at Watford.
The striker looped a header over home keeper Tomasz Kuszczak, who was making his debut following a loan move from Manchester United, to put Saints ahead.
Lambert fired in from the edge of the box before completing his hat-trick after the break from the penalty spot.
John Eustace headed wide to waste Watford's best chance of a consolation.
The home side created few other chances against a Saints side that extended their unbeaten run to six games.
The visitors were quickly into the ascendancy, Lambert taking advantage of a hesitant Kuszczak to nod in the opener from Jose Fonte's cross in the 13th minute.
Eight minutes later the Championship's leading scorer drilled in his 18th goal of the season from Adam Lallana's pull-back.
Saints should have extended their lead further shortly after the break but Tadanari Lee scooped a chance over the crossbar.
Lee had a second chance to score 20 minutes from time but Kuszczak denied him with a fine save, however the Japanese striker earned a penalty when he was pushed in the aftermath by Carl Dickinson and Lambert rifled his 19th of the season into the top corner from the spot.
Southampton boss Nigel Adkins: "Rickie Lambert is a talented, talented football player but we have won this season when Rickie's been out.
"Rickie was sent off at Brighton for a terrible refereeing decision and we lost him for three games.
"We won two of them so it's not as if we can't win without Rickie, but he is an exceptionally good football player who not only scores goals but creates goals."
Watford boss Sean Dyche: "I'm disappointed to have not performed really.
"You can't guarantee results any week and certainly not when a top-of-the-table side come to town but we pride ourselves on performance levels and we feel that today we came away from performance levels and obviously against anyone that can undo you but certainly against the market leaders.
"We wanted to make changes to freshen it up and give a bit of zip to it and we just came away from it today."