Quique Sanchez Flores' visitors remain six points from safety, while Saints climb above midweek opponents Norwich to 18th position and are only two points below Everton following a first success at St Mary's since mid-April.

The teams began the day as the division's bottom two clubs after each enduring dismal starts to the season.

Saints almost gifted the away side an opener inside two minutes but Hornets record signing Sarr fired straight at Alex McCarthy after seizing on a sloppy pass from Pierre-Emile Hojbjerg.

Senegal forward Sarr, recalled in place of Andre Gray following last week's 3-0 home loss to Burnley, was not so wasteful with his next opportunity and opened the scoring in the 24th minute with a maiden league goal since a reported £30million move from French club Rennes.

Hornets captain Etienne Capoue volleyed Kiko Femenia's throw-in over the top and Sarr outpaced Jack Stephens before coolly slotting beyond McCarthy.

For all of their early possession, Saints had little attacking threat and every loose pass or poor decision increased the considerable anxiety among the restless home support.

Boos echoed around the stadium at half-time following an uninspiring opening 45 minutes which passed without a meaningful Saints chance being created.

The hosts would have been further behind had the right leg of McCarthy not repelled a stinging volley from the lively Sarr with half an hour played.

Hasenhuttl tried to spark his team into life by introducing Sofiane Boufal and Shane Long in place of ineffectual pair Nathan Redmond and Michael Obafemi.

The substitutions almost had the desired effect when Boufal, who had been doubtful for this game after stubbing a toe in his kitchen at home, cut the ball back from the left and Long's shot was turned on to the crossbar by Ben Foster.

VAR then dismissed a Saints penalty claim after Moussa Djenepo tumbled under pressure from Capoue, while Ings earlier had another spot-kick appeal turned down by referee Michael Oliver after a tangle with keeper Foster.

Watford defender Craig Cathcart hooked off the line to deny Long as the pressure increased, before Saints top scorer Ings sparked the turnaround by prodding his seventh league goal of the season into the roof of the net from close range following fine work from Djenepo.

And the comeback was complete just five minutes later when Ward-Prowse curled into the top-left corner from 20 yards following Capoue's foul on Hojbjerg.

Southampton still had a major late scare to survive when Hornets substitute Gray volleyed wide deep into six minutes of added time, before the final whistle was greeted with cheers of relief.