Pep Guardiola has warned his Manchester City side to expect a stern examination of their Champions League credentials at St Mary's this weekend.
City ended a run of four games without a win by claiming three important points in the battle for the top-four positions with a routine victory over Hull last weekend.
But Guardiola's side still have some work to do to secure a place in Europe's top club competition next season, and the City boss is wary of the threat of Saints.
Claude Puel's men reached the EFL Cup final this season and are on course for a top-half finish. They also took a point off City with a 1-1 draw at the Etihad Stadium in a game remembered for a costly error by John Stones in October.
Guardiola said: "I think Southampton are making an exceptional season.
"They have, of course, a good manager and exceptional players in all positions. They have very good midfielders, all the strikers are so fast, they play the wingers inside - and all of them have good quality. The full-backs are guys who go up and down.
"They are a really, really good team. I was really impressed when they came here at the beginning of the season.
"Of course in that moment we came back from Barcelona, had played a lot of games in that period and we were tired.
"But of course they are a good team. It's a really good test for us."
City go into the game fourth in the table, four points clear of fifth-placed Manchester United. Their neighbours, however, have a game in hand and the two sides are still to play each other.
Guardiola has said he will decide on the day whether to play Claudio Bravo or Willy Caballero in goal.
Bravo returned for the Hull game after eight Premier League games on the bench but was criticised for allowing the Tigers to score a late consolation from the only shot they had on target.
That continued a dismal sequence for the Chilean, who was dropped after conceding six goals from six shots faced in back-to-back games against Everton and Tottenham in January.
Captain Vincent Kompany is in contention after being left out against Hull as a precaution following his long-awaited return to the side in the previous game at Chelsea.