Southampton rallied after a slow start at St. Mary's, by completing a comeback win, thanks to goals from 18-year-old Romeo Lavia and the eventual match winner from Adam Armstrong.
Raheem Sterling's goal midway through the first half came off the back of a period of significant dominance and chances created, reflecting Chelsea's control in the early stages of the game.
From a position of comfort, on the scoreline and the pitch, the visitors were caught off guard by Romeo Lavia's first Premier League goal. The strike went on to instigate a Saints revival. Adam Armstrong put the hosts ahead on the brink of the half-time interval and completely changed the momentum of the game.
Chelsea failed to make any serious ground in their attempts to get back into the game during the second half. Southampton negotiated a fairly comfortable second 45 minutes and picked up their second win of the season.
The Blues suffer their second away defeat in a row, following the disappointing defeat at Leeds. A negative goal difference after their first five fixtures leaves Chelsea on an unimpressive, by their standards, seven points at this stage of the season.
The Saints also reach the seven point mark, following tonight's result, but they will feel a lot more positively about that output, having already played half of the current 'top six'.
Story of the match
For the hosts, there was a double change in their lineup, following their most recent defeat against Manchester United. They stuck with the 4-3-3 formation, with Moussa Djenepo and Joe Aribo replaced by Romain Perraud and Ibrahima Diallo.
There were also a few changes in the Chelsea team for this fixture. Trevor Chalobah and Reece James missed out with slight injuries, while Conor Gallagher was serving a suspension, following his red card at the weekend. They were replaced in the team by Cesar Azpilicueta, Kalidou Koulibaly - returning from suspension and Hakim Ziyech.
There was also a change of system for the visitors, shifting to a 4-2-3-1 for this game.
Thomas Tuchel returned to the more familiar setting of the touchline, after his one match suspension - served against Leicester. He observed a fairly even start to the evening's action. The first ten minutes hadn't yielded much in terms of goalmouth action for either team, as they both took their time easing into the game.
Chelsea domination
Soon enough, the game burst into life and the first major chance of the evening was created. After some nice play by Jorginho and Mason Mount, the latter put through an inviting through ball towards Raheem Sterling. Chelsea's new signing couldn't get a threatening enough shot away with his weaker, left foot.
Following the cagey opening, Chelsea were beginning to assert themselves on the game. Just moments after, Ziyech's first time strike from inside the box was mishit. A word also deserves to be said about the disappointing state of the pitch at St. Mary's - visible muddy spots were prevalent across the surface, something we don't often see in the Premier League.
After 20 minutes, a neat one-two from Sterling and Ziyech created a chance for Mount to tap in from close range. However, Mohammed Salisu was aware of the danger and blocked Mount's attempt.
Only a minute later, Chelsea caught Southampton completely out of shape. A breakaway from midfield allowed for a simple pass from Ruben Loftus-Cheek to put Sterling in on goal, but Armel Bella-Kotchap was just about able to recover and prevent the England international from having an easy shot at goal.
At the 23 minute mark, the visitors got the opening goal, which had been coming. Mount's movement into the box led to him receiving the ball and coolly playing it across to Sterling. This time, Sterling was able to get on the scoresheet. His first attempt from Mount's pass was blocked, but the rebound fell kindly for him to take his time and roll the ball past Gavin Bazunu into the bottom corner.
It seemed as if this may become an ugly evening for The Saints. Ziyech's brilliant, signature long pass picked out Kai Havertz, whose powerful shot was held by the keeper.
Chelsea had won 6-0 on their last trip to Southampton in April. The amount of chances they had in the first quarter of the game suggested that it may not have been unrealistic to expect something similar, again.
Saints battle back
However, the hosts promptly managed an equaliser, from out of nowhere. On the 28th minute, a corner from the left side led to the ball dropping to Romeo Lavia at the edge of the box. The youngster produced a powerful, whipped strike, which bent around Kalidou Koulibaly and into Edouard Mendy's near corner. The keeper, maybe, should have done better after getting a touch on the ball.
Following the equaliser, Southampton seemed to get the confidence, which they desperately required. A few openings for Adam Armstrong were a sign that the game started evening back out, as Chelsea's wave of chances from earlier was calmed.
Two minutes were added on to the first half and during them, Southampton took an unexpected lead. James Ward-Prowse picked out Romain Perraud on the left side, who was just inside the Chelsea box. The Frenchmen fizzed across a perfect pass to the eagerly awaiting Armstrong. The visitor's defence was too slow to react and Armstrong, after taking a touch, produced a powerful strike with his left foot. The ball nicked off of Koulibaly and past the reach of a helpless Mendy.
As a result, The Saints headed into the interval ahead, even though it seemed, at one point in the half, that this was set to be a comfortable evening for the visitors.
Ruben Loftus-Cheek, who was later pictured with strapping around his right knee, was replaced by Mateo Kovacic at half-time.
Chelsea's 64% possession in the first 45 minutes carried over into the second half, with them having the majority of the ball, in the early stages. However, a massive chance for Southampton was the first significant action of the half. Perraud's cross picked out Mohamed Elyounoussi - who's diving header, destined for the back of the net, was blocked by Marc Cucurella and prevented a third Saints goal.
The goalscorer, Lavia, was forced off by injury in the 59th minute, replaced by Joe Aribo. Chelsea were really struggling to regain their rhythm from midway through the first half, as Southampton looked well settled into the game, by this point.
It took the visitors up until the 66th minute to create a half-decent chance, Kai Havertz was picked out in the box by Cesar Azpilicueta's cross. However, he couldn't get his head over the ball enough and the forward's effort looped well over the bar.
Both Havertz and Azpilicueta, along with Jorginho were immediately replaced by Armando Broja, Ben Chilwell and Christian Pulisic, as Chelsea switched to a three back system.
A corner nearly brought about a lead-extending goal for the hosts in the 74th minute. Salisu's low header met Ward-Prowse's delivery and it was Thiago Silva's elegant flick off the line which kept Chelsea in the game.
With time steadily fading away, Chelsea were looking fairly hopeless in their attempts to find an equaliser. With ten minutes left on the clock, Broja's headed attempt, after Sterling's cross, led to a corner, which the hosts comfortably negotiated.
The Blues did manage to apply a bit more pressure from that point onwards. But, Southampton dealt with it well enough and did not allow any real, clear chances of a goal. A late scramble in the Saints box, with Mendy advancing for his team's corner, was the all that Chelsea could muster up in the final stages of the game.
The full-time whistle blew, to the delight of Ralph Hasenhuttel, who embraced his coaching staff on the sidelines, celebrating the brilliant result.
Player of the match - Armel Bella-Kotchap
All of the Southampon starting eleven and substitutes worked together and were very well disciplined in the second half, after a nervy period leading up towards and including Chelsea's opener.
Bella-Kotchap was one of the main reasons for Chelsea's subdued second half performance. Southampton's summer signing nullified the threat of Kai Havertz - who had a quiet game from start to finish.
The 20-year-old German centre back was solid throughout the game and looks to be a shrewd pick up for The Saints at around £9 million.
A last man tackle to prevent Sterling from having a clear shot at goal in the first half was the highlight of a brilliant performance.