Manchester City lost for the ninth time in the Premier League this season as Che Adams's spectacular first Southampton goal condemned Pep Guardiola's men to a 1-0 defeat on Sunday.
Just four days after thrashing newly-crowned champions Liverpool on Thursday, City were given a reminder of why they are 23 points behind the Reds at the top of the table as defensive mistakes and a host of missed chances cost them at St. Mary's.
Adams caught City goalkeeper Ederson stranded well outside his area to score the only goal on 16 minutes as Saints then somehow held out to mathematically secure their Premier League status for another season.
Guardiola made six changes from the rout over Liverpool with Kevin De Bruyne among those dropping to the bench.
Even without the brilliant Belgian, City still created enough chances to win comfortably but were only denied by some inspired goalkeeping by Alex McCarthy and wayward finishing.
Southampton were given something to hold on to on 16 minutes when Stuart Armstrong robbed Oleksandr Zinchenko and Adams ended his 30-game wait to score a goal for Southampton in spectacular style by catching Ederson out of his goal from over 40 yards out.
The Saints goal then lived a charmed life as Gabriel Jesus fired over with the goal gaping from Joao Cancelo's cross, Fernandinho struck the post and McCarthy made the first of a string of saves from David Silva's powerful header.
Ralph Hasenhuttl's men were still causing a threat themselves on the counter-attack and Danny Ings uncharacteristically wasted a golden chance to make it 2-0 and close on Leicester's Jamie Vardy in the race for the Golden Boot by shooting over with just Ederson to beat.
City continued to dominate after the break as McCarthy spread himself brilliantly to save from Silva before Bernardo Silva's goalbound effort was deflected wide.
De Bruyne and the in-form Phil Foden were introduced on the hour mark, but still the hosts' defence held firm to keep City scoreless for just the fourth time this season.
Saints were marooned in the bottom three when the sides last met in a 2-1 City win in November, a week after they suffered a Premier League record 9-0 home defeat to Leicester.
But the Southampton board's backing of Hasenhuttl, who recently signed a contract extension to 2024, has been fully justified as the Austrian's side climb to 13th and could yet be the beneficiaries of City's misfortune.
Southampton are just five points off eighth place which may be enough for a place in next season's Europa League should City's appeal against a two-season ban from European competition not be successful later this month.