Southampton beat Premier League leaders Everton 2-0 at St Mary's Stadium on Sunday with goals from James Ward-Prowse and Che Adams as Carlo Ancelotti's side became the final club to lose a game this season.
Everton remain top of the standings with 13 points and a healthier goal difference than city rivals and reigning champions Liverpool but Southampton moved up to fifth with their third win in four matches.
"We knew it was a good challenge for us, they came here unbeaten and we didn't like that feeling. Our confidence was high," Ward-Prowse told Sky Sports.
"We were aggressive, we believe in what we are doing and we have the quality to go and score goals... This gives us a lot of confidence, we weren't really troubled."
Southampton took the lead in the 27th minute when Ward-Prowse played a neat one-two with Danny Ings, who provided the through ball for his fellow Englishman to fire a shot past Jordan Pickford.
Ings turned provider again eight minutes later when he crossed from the left flank to find Adams at the far post and the unmarked striker took his time to pick his spot to beat Pickford with a strike that deflected off Gylfi Sigurdsson.
With about 20 minutes to go, Everton were reduced to 10 men when defender Lucas Digne was sent off for a straight red card after he fouled Kyle Walker-Peters and Ancelotti could not bring on another defender having already made three substitutions.
Everton hardly tested Alex McCarthy in Saints' goal, with striker Dominic Calvert-Lewin drawing his first blank of the season.
The closest the visitors came to scoring was when Sigurdsson's first-half effort from outside the box crashed into the crossbar while James Rodriguez had a tame effort in the box saved.
"It was a tough win against a strong side but we had good game management... It was one of our best performances at home," Saints manager Ralph Hasenhuettl said.
"We played through the lines, the second goal was beautiful and we really calmed the game down. I didn't see a lot of chances for Everton today."