Adam Lallana scored against his former club as Brighton dealt Southampton boss Nathan Jones a 3-1 defeat on his first Premier League home game in charge at St Mary's Stadium.
Southampton were booed off by their own supporters at half-time after a header by former captain Lallana and an own goal by left-back Romain Perraud heaped more misery on Jones' relegation-threatened side.
Solly March then added a third for the Seagulls in the second half, thumping a shot into the top corner from 20 yards out that Saints goalkeeper Gavin Bazunu had no chance of saving.
A penalty was awarded to Southampton in the 73rd minute after Samuel Edozie ran into Pascal Gross inside the area, and while Sanchez saved James Ward-Prowse's initial effort, he could not keep out the headed rebound by the home skipper.