Chelsea peppered the Southampton goal throughout the second half but were unable to find a way past substitute goalkeeper Kelvin Davis, leading to a defeat that drops Rafael Benitez's side to fourth.
Rickie Lambert celebrated his new Southampton contract in some style today, smashing home an exquisite free-kick to secure all three points from a hard-fought encounter with Chelsea.
Making his 500th career league appearance, the Liverpudlian curled home a spectacular 25-yard free-kick to rack up his 14th league goal of the campaign - more than any other English player in the Premier League.
Saints had gone ahead through another in-form striker Jay Rodriguez, who collected a neat one-two from Steven Davis before slotting home in the 23rd minute.
John Terry was allowed to level unmarked from a corner 10 minutes later, only for Saints to hit back immediately through Lambert's fine strike.
Rickie Lambert's 35th-minute free-kick proved enough to seal a 2-1 win at St Mary's after an early opener from Jay Rodriguez had been cancelled out by a header from John Terry.
Chelsea's shock loss allowed Tottenham to leapfrog into third place with a 2-1 win at Swansea.
Brighton and Hove Albion have announced a mouth watering friendly fixture ahead of the new season, playing Champions League winners Chelsea.
Gus Poyet will go head-to-head with his former Blues team-mate and close friend Roberto Di Matteo, who recently signed a two-year deal at Stamford Bridge in the wake of their European success.
Capacity at the Amex for the game on Saturday, August 4, will be limited to 18,000 due to the ground's expansion, adding 8,000 more seats.