Substitute Dusan Tadic fired the only goal as Southampton recorded a first league win over Manchester United for 27 years on Sunday, climbing above them into third in the Premier League.
A superbly disciplined display earned Saints a 1-0 win at Old Trafford that ended United's 10-match unbeaten run.
After taming United's attacking options, Southampton struck in the 69th minute when Tadic calmly converted a rebound.
The south coast club moved up to third on 39 points from 21 matches, while United drop to fourth on 37 points.
"If you play against a big side you need spirit and quality but you also need belief in yourself and your team-mates and we showed that today," Saints manager Ronald Koeman told Sky Sports.
"We have had good organisation all season, we have 11 clean sheets in 21 games and that is key to being successful."
Earlier, Alexis Sanchez scored twice and created a third as Arsenal beat Stoke City 3-0 at the Emirates Stadium to move above Tottenham Hotspur into fifth spot with 36 points.
United struggled to break down a well-organised Southampton defence
With just a point separating the two sides going into the match, the first half was short of thrills as Southampton restricted United's flow in midfield.
Southampton right back Nathaniel Clyne fired a shot wide in the second minute before Angel Di Maria missed the target from distance as both sides failed to spark.
For all their attacking riches, United struggled to break down a well-organised Southampton defence with Wayne Rooney often left frustrated by a lack of opportunities.
The home crowd were already getting restless when the visitors took the lead.
Tadic who kept his composure to fire past David De Gea
After a silky move Saints striker Graziano Pelle struck a shot against the base of the post and the ball rebounded to Tadic who kept his composure to fire past David De Gea.
Tadic was then shown a yellow card for ripping off his shirt, although his joy was understandable.
Juan Mata sent a volley wide as United squandered a number of chances late on as Louis Van Gaal's side tasted defeat at home for the first time since the opening day of the season.
Sublime Sanchez
Arsenal were 3-0 down at halftime against Stoke in the reverse fixture at the Britannia Stadium, but the Gunners were in electric form at the Emirates.
After they were twice denied brilliantly by Stoke goalkeeper Asmir Begovic, defender Laurent Koscielny was afforded far too much space in the Stoke penalty area to nod home Sanchez's pinpoint cross in the sixth minute.
Arsenal's explosive start was marred by a dislocated shoulder suffered by French defender Mathieu Debuchy, but Sanchez added a brilliant second with a clever near-post finish following a neat build up with Tomas Rosicky.
‘Very important that the three points is with us’
Chilean Sanchez, so often Arsenal's go-to man this season, wrapped up the points for the Gunners shortly after the break when his fortuitous deflected free kick squirmed past a despairing Begovic.
"It was very important to start the game well. We remember the game at Stoke, after 20 seconds we conceded a goal," Koscielny told Sky Sports.
"We were very focussed and it's very important to score the (early) goal and very important that the three points is with us."
