Manolo Gabbiadini continued his hot scoring streak as Southampton bounced back from last week's Wembley disappointment to see off Watford in a flurry of late goals.
The £14million January signing from Napoli became the first Saints player to have scored in all of his first four games and his goal in this one - his side's crucial third - took his tally to six.
The impressive Nathan Redmond supplied two, with Dusan Tadic also on the scoresheet to deny Watford, who had taken an early lead through Troy Deeney and looked on course for at least a point when Stefano Okaka made it 2-2 with 11 minutes remaining.
Saints had spent the week nursing the hurt served up by Zlatan Ibrahimovic's late EFL Cup final winner for Manchester United on an occasion when Gabbiadini scored twice.
Southampton manager Claude Puel hailed the Italian - who he insisted actually had seven goals in English football to his name as he had another wrongly ruled out for offside at Wembley.
"It was difficult for him today because he played against three centre backs," he said. "But he continues to score and its important. He has scored seven goals - he scored three goals against Manchester - and of course it's fantastic work.
"We stayed with the good character we showed against Manchester United, and showed the capacity to come back in the Premier League.
"It was a difficult to prepare after the final, and after the disappointment. But after we conceded an early goal we stuck to our plan to play good football."
The early goal belonged to Deeney, who had scored after just three minutes in Watford's previous game, a 1-1 draw at home to West Ham. He repeated the trick almost to the second in this one with a first time effort from Okaka's lay-off that curled beyond Saints keeper Fraser Forster.
That fine left-footed strike brought the Watford captain's goal tally for the season to nine, five of which had arrived in his seven most recent games. It also made him the club's joint fourth highest goalscorer with 105, level with Cliff Holton, who had two spells as a lower league Hornet between 1958 and 1966.
Southampton's equaliser, arriving in the 28th minute through Tadic, was the result of poor defending with a tinge of controversy added. Tadic, prompted by Redmond's flick, surged forward but was stopped by a wall of yellow that somehow recycled the ball straight back to him as Redmond waded in. The Serb launched a low shot into the far corner that bypassed Redmond, stuck in an offside position but not directly in the goalkeeper's vision.
Gabbiadini was booked for going in high on Valon Behrami and Gomes soon had to rush out of his box to head away to stop the Italian profiting from a one-two with Tadic. The Brazilian then had to react to stop Tadic squeezing in another from a tight angle.
There was just enough time before the break for Redmond to start and finish a fine move to put Saints ahead. The former Norwich man used his pace to leave Behrami trailing and was in the right place on the right of the box to fire across Gomes and in off a post after Tadic, set up by Ward-Prowse, had pulled the ball back. It was deserved.
Saints continued where they had left off after the break and Gomes pulled off a super tip over to deny Tadic from a corner. The Brazilian then came up with a fine reflex parry to deny Maya Yoshida, who had gone up for a free-kick.
With 20 minutes remaining, Watford sent on Isaac Success, and after Forster had blocked Okaka's shot with his legs, it was the substitute who set up the Italian for a 79th minute leveller that was smashed past the England goalkeeper from seven yards out.
The joy was shortlived however as Gomes could not hold on to a drive by substitute Sofiane Boufal and Gabbiadini, marginally onside, tucked away the rebound.
Redmond made sure of victory with his second two minutes later, cutting inside from the left to unleash a rising drive that Gomes perhaps should have saved.
Substitute Abdoulaye Doucoure bundled in a Watford third in stoppage-time but it was only a consolation.
Watford manager Walter Mazzarri added: "I want to underline the fact that we never gave up and managed to score in the last minute to make it 4-3 and this is something very positive.
"But I don't want it in the head of my players that they are already safe from relegation. No game in the Premier League is written, anything can happen. We need to learn how to close out games."
Watford (4-1-3-2): Gomes; Kaboul, Prodl, Britos. Holebas; Behrami; Niang, Cleverley, Holebas, Capoue (Success 70); Deeney, Okaka (Doucoure 89). Subs (not used): Arlauskis, Mariappa, Cathcart, Zuniga, Stewart.
Southampton (4-3-3): Forster; Cedric, Yoshida, Stephens, Bertrand; Davis, Romeu, Ward-Prowse (Boufal 78); Tadic, Gabbiadini (Long 85), Redmond. Subs (not used): Hassen, Clasie, Rodriguez, Cacares, McQueen.
Referee: Jonathan MossReuse content