After going all the way to beat Jose Mourinho’s men last time out, many had predicted that Chelsea will have a ride over the Saints in the Boxing Day tie at Stamford Bridge.
However, that was not the case as Michael Obafemi, an Irish striker of Nigerian descent, gave Southampton a first-half lead before Nathan Redmond sealed victory for the visitors with his 73rd-minute effort.
Obafemi with his goal at 19 years and 173 days became the youngest player to score a Premier League goal away at Stamford Bridge against Chelsea.
With the massive victory recorded in London, Southampton have taken a big leap away from the relegation zone as they have moved to 14th spot with 21 points after 19 games.
On their part, Chelsea are still fourth on the log but they only have a three-point cushion ahead of fifth-place Tottenham Hotspur who started the day with a 2-1 comeback win over Brighton.
With this, Chelsea have now lost back-to-back home Premier League matches for the first time since November 2011 under Andre Villas-Boas.
In the other Boxing Day games, Carlo Ancelotti started his reign as Everton coach on a winning note as his team squeezed out a lone goal victory over Burnley.
The Toffees were made to sweat for Thursday’s three points as they got their only goal ten minutes before full time through Dominic Calvert-Lewin.
While Ancelotti hit the ground running, it was not the same for Arsenal’s Mikel Arteta as the Gunners could only secure a 1-1 draw against Bournemouth.
Dan Gosling gave the hosts the lead in the first half before Pierre-Emerick Aubameyang restored parity in the second half
Aubameyang has now scored 11 of Arsenal’s last 16 away league goals, including seven of their last nine on the road in the Premier League.
Elsewhere, Crystal Palace piped city rivals West Ham 2-1 while Aston Villa halted their poor run with a 1-0 win over fellow relegation battlers Norwich City.