Southampton handed Chelsea their second defeat of the season as they roared to a much-deserved 2-1 victory at St Mary's on Tuesday.
Raheem Sterling put Chelsea ahead early on but first-half strikes from Romeo Lavia and Adam Armstrong put the Saints ahead and handed Ralph Hasenhuttl's side three points for which they were more than good value.
It was a lively start from both sides, with Southampton perhaps getting the better of the opening exchanges, but Chelsea should have taken the lead 13 minutes in when Sterling found some space in the box but could only send a tame effort towards Gavin Bazunu's goal.
Sterling then saw a potential one-on-one prevented superbly by Armel Bella-Kotchap, but the Chelsea pressure eventually led to the breakthrough after 23 minutes as the summer signing kept his composure to slot home from close range.
It could have been 2-0 moments later after a glorious Hakim Ziyech ball fed Kai Havertz, who found himself at a tight angle and could only send his strike right at Bazunu.
Against the run of play, Lavia thundered home an equaliser shortly before the half-hour mark, firing a rocket from the edge of the box that nestled in the bottom corner.
The goal sparked Southampton into life and two tame strikes from Armstrong served as reminders to Chelsea that the hosts wanted more than a point from this game, and they deservedly got themselves in front in first half stoppage time when Armstrong fired past Mendy from ten yards.
Chelsea rediscovered their energy immediately following the restart, but Southampton were inches away from grabbing a third five minutes in. Marc Cucurella had to block Mohamed Elyounoussi's header off the line, before his second effort was gratefully swallowed up by Mendy.
Just as quickly as they came to life, Chelsea faded away once again and Thomas Tuchel responded with a triple substitution that saw Christian Pulisic and former Southampton loanee Armando Broja enter the fray.
The Blues needed another goal-line clearance to stay alive in this one, with Thiago Silva doing the honours this time to deny Mohammed Salisu, with Chelsea hanging on defensively at times.
As the clock ticked on, Chelsea tried to throw themselves forwards in search of an equaliser, but in reality, they never looked like finding one. The quality in the final third was nowhere near good enough and Southampton were more than happy to blast their clearances away until the full-time whistle.
Southampton vs Chelsea player ratings
1. Southampton (4-2-3-1)
GK: Gavin Bazunu - 7/10 - Wasn't overly busy but looked confident whenever called into action.
RB: Kyle Walker-Peters - 7/10 - A lively threat down the right side. Flew up and down the pitch.
CB: Armel Bella-Kotchap - 8/10 - Made an excellent tackle to deny Sterling and looked dominant at everything he did.
CB: Mohammed Salisu - 7/10 - A nervy start but recovered well. Too strong for Chelsea's forwards.
LB: Romain Perraud - 7/10 - Nice vision for Armstrong's go-ahead goal. A real menace down the left.
DM: Ibrahima Diallo - 6/10 - Did a good job of stifling Chelsea's movement in midfield.
CM: Romeo Lavia - 8/10 - A great goal and some dominant midfield work. Really enjoyed his evening.
RM: Adam Armstrong - 7/10 - Doing well out on the wing. Made some dangerous runs and arguably should have had more than his one goal.
AM: James Ward-Prowse - 7/10 - Found himself in acres of space in this new advanced role. His high press was far too much for Chelsea.
LM: Mohamed Elyounoussi - 7/10 - Also pressed with immense energy. Helped the Saints get back on top.
ST: Che Adams - 8/10 - A massive problem for Silva. Used his physicality to his advantage time and time again. His energy was sensational.
SUB: Joe Aribo (60' for Lavia) - 6/10 - A nice, physical presence in midfield.
SUB: Moussa Djenepo (73' for Perraud) - 6/10 - Not as impactful as Perraud but did his job of keeping things calm.
SUB: Lyanco (86' for Armstrong) - N/A
Manager: Ralph Hasenhuttl - 9/10 - Elyounoussi and Armstrong as wingers, Ward-Prowse as an advanced midfielder - both excellent decisions. Neutralised Chelsea perfectly.
2. Chelsea (4-3-3)
GK: Edouard Mendy - 5/10 - Looked like he could have done better for Lavia's equaliser.
RB: Cesar Azpilicueta - 4/10 - His limited movement is becoming a real problem. Offered nothing down the right.
CB: Thiago Silva - 4/10 - Wanted no part of Adams. Looked terrified to compete in the air.
CB: Kalidou Koulibaly - 5/10 - Some really nice passing but a lack of cover in this formation really exposed his ageing legs.
LB: Marc Cucurella - 6/10 - Had to do all the running for his entire defence. Only so much he could do.
CM: Ruben Loftus-Cheek - 4/10 - Seemed flustered back in his natural midfield role. Off at half-time with an injury.
CM: Jorginho - 3/10 - Offered no cover for his defence whatsoever. An embarrasing dive after the ball was kicked towards his face.
CM: Mason Mount - 3/10 - Made a few nice runs but completely lost control of the game after Lavia's equaliser.
RW: Hakim Ziyech - 4/10 - The odd nice pass but nowhere near impactful enough for a player trying to push for more minutes.
ST: Raheem Sterling - 5/10 - Put a shift in but was a little sloppy in front of goal. Good composure for his opener.
LW: Kai Havertz - 2/10 - Good for the first ten minutes and then absolutely woeful for the next 50 before Tuchel rightly dragged him off. Hindered more than helped.
SUB: Mateo Kovacic (46' for Loftus-Cheek) - 5/10 - Immediately made a positive impact but still couldn't change things.
SUB: Christian Pulisic (66' for Havertz) - 5/10 - Similarly impactful straight off the bench but soon faded.
SUB: Ben Chilwell (66' for Jorginho) - 3/10 - Almost looked like he was trying not to get involved.
SUB: Armando Broja (66' for Azpilicueta) - 6/10 - Starved of service but looked a genuine threat when he did get the ball. Entitled to feel as though he deserves more minutes.
Manager: Thomas Tuchel - 3/10 - A four-man defence with three players over 31, shielded by Jorginho, felt like tactical suicide. Obviously struggling with injuries but it's his job to find effective solutions.