The Blues were looking to close the gap on the top three in the table -- with Southampton on a bad run of form with six straight defeats in the Premier League since the win over Liverpool -- but their task became more difficult when Takumi Minamino's composed finish put the home side ahead against the run of play.
It was a confident start from the visitors who dominated proceedings from the first whistle. The visitor's injected some much-needed intensity into their play after the break which ensured they remain unbeaten under Thomas Tuchel.
The wasteful Blues produced just one shot on target in the opening 38 minutes -- a feeble 40-yard effort from defender Antonio Rudiger -- and they were punished by Minamino's suckerpunch on the counter which could see Tuchel's men end the weekend outside the top four.
Manager rating out of 10
7 -- Surely the Premier League's noisiest manager on the sidelines, Chelsea have looked a dominant force under Tuchel's leadership. Despite that, he cut a frustrated figure for large spells, reflected by his decision to bring on -- and then off -- winger Callum Hudson-Odoi in the second half.
Player ratings (1-10; 10 = best. Players introduced after 70 minutes get no rating)
GK Edouard Mendy, 6 -- You can't really hold him responsible for the opening goal, but he didn't really impose himself either and was floored by Minamino's tidy footwork. He was more assured to gather a James Ward-Prowse corner, but was thankful to see Vestergaard's effort shake the bar.
DF Reece James, 6 -- He just seems to take the Blues up a gear when he's available. It was another excellent start from the young full-back who terrorized his opponents with a series of surging runs, but he blazed a good opportunity over the bar late on.
DF Cesar Azpilicueta, 5 -- The Chelsea skipper barely had anything to do for half an hour, but Minamino sat him down before coolly rolling the opener past Mendy.
DF Antonio Rudiger, 5 -- His heart was probably in his mouth when Mendy didn't seem to be paying all that much attention as he nodded the ball back to his keeper. His 40-yard strike was ambitious at best and he was caught out of position for the Southampton goal.
DF Kurt Zouma, 6 -- Enjoyed a quiet start with Southampton attacks a rarity, but he did his best to catch the eye with a nice switch of play to pick out Marcos Alonso on the left and drew a smart save from McCarthy with a powerful header at the back post.
DF Marcos Alonso, 7 -- Tuchel promised fresh starts after his arrival and Alonso has benefitted more than anyone. He's been good value too and lashed a powerful strike wide of the far post before fizzing in a low cross that almost found Timo Werner.
MF N'Golo Kante, 6 -- Does he ever stop running? Another gut-busting performance from the tireless midfielder who probably covered every blade of grass twice, but even he couldn't snuff out Redmond's pass that sent Minamino on his way to open the scoring.
MF Mateo Kovacic, 6 -- Like Kante, he imposed himself well as you'd expect which allowed Chelsea to dictate proceedings. He fed Alonso on the touchline with a neat pass too as the Blues continued to find joy on the left flank.
MF Mason Mount, 8 -- Chelsea's star man by some distance. It was a tireless performance from the tenacious midfielder who has quickly won over Tuchel. He was brilliant in the second half and picked himself up off the turf to calmly roll past McCarthy from 12 yards.
FW Tammy Abraham, 5 -- The forward has looked good in recent times and started brightly again here, but he was thwarted by some alert defending from Mohammed Salisu as he looked to attack Saints' box and had to be withdrawn through injury at the break.
FW Timo Werner, 5 -- An afternoon of near misses for the pacey forward, who produced a handful of driving runs and dangerous cut-backs, but they failed to find a blue shirt and Tuchel wasn't best pleased as his overhit switch of play to Alonso crept out of play.
MF Jorginho, NR -- A typically industrious outing, but the midfielder clearly wasn't in the mood to take chances as he professionally hauled down his man on halfway to abruptly halt a Southampton counter-attack.
FW Callum Hudson-Odoi, 5 -- Caused problems off the bench with his quick feet and had a big hand in the leveller when he picked out Mount who was clattered by Ings, only to be hauled off himself in place of Hakim Ziyech half an hour later. Brutal.
FW Hakim Ziyech, NR -- If Hudson-Odoi was already disappointed, Ziyech's underpar contribution will have done little to lift his spirits as the winger was brushed off the ball by Bertrand with his first involvement.