Eddie Nketiah and Joe Willock were both on target as we secured a welcome first win since the Premier League restarted.
The opener came when Nketiah seized on a goalkeeping error. Alex McCarthy tried to find a defender and Nketiah closed down well and anticipated the pass, blocking the ball back towards goal and dashing past the keeper to apply the simplest of finishes.
It was Nketiah's fourth goal of the season, and his six in 11 starts across all competitions if you include his loan spell at Leeds.
Southampton had actually controlled the early stages of this game but we gradually exerted a grip, and always looked a threat with our pace on the counter-attack.
Pierre-Emerick Aubameyang latched onto one through-ball to crash a close-range shot off the underside of the bar, and one delightful move ended with Nketiah finding the net from Bukayo Saka's cross - only for the offside flag to cut short any celebrations.
Nketiah got what he wanted with that opening goal and almost scored a second when Nicolas Pepe found him inside the box.
We had a few scares after the break, not least when Emi Martinez had to made a brilliant one-handed save to deny Shane Long - and then quickly smother the loose ball.
But Jack Stephens was sent off for bringing down Aubameyang as he raced through on goal, and from the resulting free-kick we settled the match.
Alex Lacazette's initial effort hit the wall, his follow-up was parried by McCarthy - and Willock pounced to score from close range.
HOW IT HAPPENED
Five THINGS WE NOTICED
WHAT IT MEANS
Mikel Arteta made four changes to the side for this second successive trip to the south coast.
Granit Xhaka returned from a sprained ankle to take his place in midfield, Nketiah replaced Lacazette up front and Emi Martinez came in for the injured Bernd Leno.
Sokratis was back on the bench after injury he was joined there by young midfielder Matt Smith and goalkeeper Matt Macey.
Matteo Guendouzi was not in the squad and the injured Gabriel Martinelli also missed out.
Our Emirates FA Cup campaign resumes on Sunday with a sixth-round trip to Sheffield United.
We're back in Premier League action next Wednesday with our first home game since the restart - against Norwich City.
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