Southampton came from behind to stun Chelsea 2-1 as Adam Armstrong's goal dealt Thomas Tuchel's men their second defeat of the Premier League season on Tuesday.
Chelsea hit the front through Raheem Sterling after 23 minutes, but Romeo Lavia's powerful long-range strike dragged Southampton level five minutes later before Armstrong put the hosts ahead on the stroke of half-time, reports DPA. With Reece James missing through illness, Chelsea struggled to create throughout a disjointed performance, and Kai Havertz squandered their best chance to level after the break.
The result saw Ralph Hasenhuttl's men draw level with Chelsea in the early Premier League table after a lively display, as the Blues' unconvincing start to the campaign continued.
Sterling produced a tame attempt when presented with Chelsea's first chance 13 minutes in, but he made amends to hand the Blues the lead, turning to poke home at the second attempt following good work from Mason Mount.
Chelsea's lead did not last long, however, with Lavia latching onto Havertz's clearance to open his Premier League account in spectacular fashion by drilling a vicious 18-yard strike past Edouard Mendy.
Southampton then hit the front through a well-worked goal as half-time approached, Armstrong controlling Romain Perraud's cross before composing himself and firing into Mendy's top-left corner.
Marc Cucurella kept Chelsea in the contest with an incredible goal-line block from Mohamed Elyounoussi's header after the break, before Havertz headed over from six yards when left unmarked as he met Cesar Azpilicueta's cross.
That represented Chelsea's clearest opportunity to find a leveller, with Thiago Silva forced to clear Mohammed Salisu's 73rd-minute header off the line as Saints went close to a third.
Chelsea welcome West Ham to Stamford Bridge for a London derby on Saturday, while Southampton go to Wolves on the same day.
Meanwhile, Thomas Tuchel has told Chelsea to "toughen up" in the wake of the chastening defeat.
"We are not tough enough to win these matches at the moment, away from home," said Tuchel.
"It's a bit the same story like in (the 3-0 defeat at) Leeds. In Leeds we thought the problem was that we did not score in the first 20, 25 minutes. They put us off balance with two goals in two minutes at Leeds and we could not find answers.
"Today we scored so we thought everything was okay, but one set-piece again, one sloppy defending again, and we are off balance. And today we did not react to it; we were lucky not to concede the third goal.
"We were not tough enough as a team to react or to push it over the line for us."
Asked to review the goals Chelsea conceded, Tuchel continued: "Soft, soft, soft. What stops that? Stop it by pure mentality. Stop it by pure mentality. There is no superiority for the opponent, there is no need to get shots away, there is no need. Just toughen up as a team. And show the mentality. This is today my assessment.
"I don't like to talk about it normally because you cannot prove it with the data, body position or with the tactical position in defeat. But it's like this. Both goals are cheap goals, soft goals, and should not happen if you expect to win a Premier League match, a night game and an away game.
"With our demands and the ambitions we have, it's even more important if key players are missing that we need to show a different mentality. It's not enough at the moment to be defeated in Southampton. There's no shame in it, we are humble enough to accept that we can lose football matches.
"But it's too easy to push us off the track, it's too easy to win challenges, it's too easy to bully us."
Southampton were left to toast a famous win, with a raucous crowd delighting in upending the 2021 Champions League winners.