Chelsea have lost twice in their opening five Premier League games for just the second time in 27 seasons after falling to a 2-1 defeat at Southampton.
The Blues, who were beaten 3-0 at Leeds earlier in August, started well at St Mary's and took the lead courtesy of Raheem Sterling's third goal in four days.
But Southampton hit back through Romeo Lavia and Adam Armstrong.
Highlights: Southampton 2-1 Chelsea
Sterling had already had a shot from inside the box before he scored Chelsea's opening goal on 23 minutes.
Chelsea's opener came from a sweeping move that saw Kai Havertz find Mason Mount, who squared for Sterling.
Sterling was initially denied by a strong block from Romain Perraud but the England forward was first to the loose ball to slot home from six yards.
Chelsea led for just five minutes. A James Ward-Prowse corner was only half-cleared, allowing Lavia to score powerfully from just outside the 18-yard box.
Lavia, aged 18 years and 236 days, is now the fourth youngest Southampton scorer in Premier League history.
The Saints completed the turnaround in first-half stoppage time.
Perraud fed Armstrong, whose shot beat Edouard Mendy via a deflection off Kalidou Koulibaly.