Match ends, Southampton 1, West Ham United 2.
Felipe Anderson scored twice in six minutes as West Ham came from behind to beat Southampton at St Mary's Stadium and move up to ninth in the Premier League table.
After a quiet first half, the hosts took the lead in scrappy fashion through Nathan Redmond after Lukasz Fabianski had parried his initial effort.
But West Ham were only behind for three minutes before Anderson's 20-yard low strike beat Alex McCarthy.
The third goal - all in nine minutes - came again from Anderson, who powerfully shot home after being released by Michail Antonio to put the Hammers ahead.
Southampton pushed for an equaliser, but Fabianski and the visiting defence held firm to secure West Ham their fifth win in six games.
Injury-hit Hammers win again
For Manuel Pellegrini's side to be in the top half of the table at the halfway point of the season is a remarkable achievement, considering they lost their opening four league matches and have been ravaged by injuries.
Manuel Lanzini, Winston Reid, Carlos Sanchez, Andriy Yarmolenko and Ryan Fredericks are all long-term absentees, with Jack Wilshere, Marko Arnautovic, Javier Hernandez, Fabian Balbuena and Pablo Zabaleta also unavailable.
The Hammers' injury problems led to Pellegrini only naming six substitutes against Southampton and two of those - 21-year-old forward Xande Silva and 18-year-old midfielder Conor Coventry - had not featured in any Premier League match.
Despite that the visitors had the best opportunity of the opening half but Lucas Perez scuffed a shot wide after good work from overlapping full-back Aaron Cresswell, while Robert Snodgrass shot wide just before half-time.
Southampton took the lead in the 50th minute when a combination of Redmond and Angelo Ogbonna bundled the ball over the line, before two goals in six minutes from Anderson turned the game around.
It continues a fine December for Pellegrini's team that has also seen them beat Newcastle, Cardiff, Crystal Palace and Fulham, with their only slip-up being their 2-0 home loss to Watford on Saturday.
They were 12th before the game, but the three points has lifted them above Bournemouth, Wolves and Watford and level on points with eighth-placed Everton.
Two wins, two losses for Hasenhuttl
The match was Ralph Hasenhuttl's fourth game in charge of the Saints since the club sacked Mark Hughes earlier this month and the Austrian has now won two and lost two in England.
His side had won their past two matches - against Arsenal and Huddersfield - and could have taken a third-minute lead after Danny Ings jinked past Ogbonna and Antonio but shot into the side netting from a tight angle.
In a frantic nine-minute second-half period, Southampton took the lead, West Ham equalised and then the Saints had a great chance to retake the lead - but Fabianski made a fine one-handed save to deny Stuart Armstrong.
Moments later, following a Saints corner, the hosts thought they should have had a penalty when Declan Rice held on to Jan Bednarek, but referee Craig Pawson did not give the decision and the visitors countered, leading to Antonio setting up Anderson for their second goal.
Southampton are in their seventh successive season in the top flight and their battle to avoid relegation went down to the final game of the 2017-18 campaign.
After 19 of 38 league matches in 2018-19, they are now 16th, only three points clear of 18th-placed Burnley - and they face Manchester City and Chelsea in their next two games.
More dropped points after leading for Southampton - the stats
- Southampton have dropped 15 points from winning positions in the Premier League this season; the most of any team in the competition, with 12 of those coming in home games.
- West Ham have won five of their last six games in the Premier League (lost one) - their previous five wins in the competition came over a run of 20 games between April and November.
- This was West Ham's first Premier League away win after conceding the opening goal since May 2017, when they won 2-1 against Burnley at Turf Moor.
- Southampton have suffered back-to-back Premier League defeats against West Ham for the first time since October 2001 (a run of four).
- West Ham have won 15 points in the Premier League in December (P6 W5 D0 L1); their most in a single month of a season in competition history.
- After scoring just once in his opening 10 Premier League appearances, West Ham midfielder Felipe Anderson has scored seven goals in his past nine.
- Southampton winger Nathan Redmond has scored in back-to-back Premier League games for the first time since October 2016.
- There were just 168 seconds between Southampton opening the scoring and West Ham equalising through Felipe Anderson's strike.
Man of the match - Felipe Anderson (West Ham)
'We were not so fresh' - what they said
Southampton boss Ralph Hasenhuttl told BBC Sport: "The way we lost it was too easy, we didn't play such a good game.
"If you give it away like we did then you don't deserve to win. We were not so fresh and not so speedy, [or] sharp to win the second balls.
"We ran a lot without the ball, at half-time we changed a few things put early pressure on and scored the first goal. [Their goals were] too easy, one from outside the box and one from a corner we had."
West Ham boss Manuel Pellegrini, speaking to Sky Sports: "All the credit [to the players]. We played a very good game from the first minute to win the game and we were very balanced in the whole game.
"We worked mostly in the opposition side and we defended very well. They didn't have too many options. It's important we drew level immediately and scored the winning goal."
On match-winner Felipe Anderson: "Of course he is a different player, a player that in every game makes important plays - he will continue improving.
"He still loses too many balls, maybe because he must understand the Premier League is difficult if you keep the ball in your feet."
It doesn't get any easier for Southampton, who on Sunday host a Manchester City side looking to make amends for back-to-back defeats.
On the same day, West Ham host Burnley, who were beaten 5-1 at home by Everton on Wednesday.
- 35BednarekBooked at 83mins
- 3YoshidaSubstituted forAustinat 80'minutes
- 18LeminaSubstituted forElyounoussiat 87'minutes
- 33TargettBooked at 66mins
- 9IngsSubstituted forLongat 62'minutes
- 45DianganaSubstituted forCarrollat 55'minutes
- 11SnodgrassSubstituted forMasuakuat 90+2'minutes
- 8Felipe Anderson
- 27PérezSubstituted forNobleat 78'minutes
- 32Costa Silva
- Craig Pawson
- Shots on Target
Second Half ends, Southampton 1, West Ham United 2.
Corner, West Ham United. Conceded by Yan Valery.
Attempt blocked. Declan Rice (West Ham United) right footed shot from the centre of the box is blocked.
Attempt saved. Felipe Anderson (West Ham United) right footed shot from a difficult angle on the right is saved in the bottom right corner. Assisted by Andy Carroll.
Substitution, West Ham United. Arthur Masuaku replaces Robert Snodgrass.
Felipe Anderson (West Ham United) wins a free kick in the defensive half.
Foul by Nathan Redmond (Southampton).
Corner, West Ham United. Conceded by Alex McCarthy.
Attempt saved. Felipe Anderson (West Ham United) right footed shot from a difficult angle on the left is saved in the bottom left corner. Assisted by Lukasz Fabianski.
Substitution, Southampton. Mohamed Elyounoussi replaces Mario Lemina.
Offside, Southampton. Stuart Armstrong tries a through ball, but Shane Long is caught offside.
Attempt missed. Michail Antonio (West Ham United) left footed shot from outside the box misses to the left. Assisted by Mark Noble.
Attempt blocked. Declan Rice (West Ham United) left footed shot from the centre of the box is blocked.
Jan Bednarek (Southampton) is shown the yellow card for a bad foul.
Andy Carroll (West Ham United) wins a free kick in the attacking half.
Foul by Jan Bednarek (Southampton).
Attempt blocked. Charlie Austin (Southampton) right footed shot from the left side of the box is blocked. Assisted by Stuart Armstrong.
Substitution, Southampton. Charlie Austin replaces Maya Yoshida.
Substitution, West Ham United. Mark Noble replaces Lucas Pérez.