Ronald Koeman has described Leicester City's season so far as an amazing achievement, with the team edging closer to winning the Premier League.
Saints' opponents on Sunday are currently five points clear at the top of the division, with seven games left to play in the season.
It is certainly a far cry from when Koeman last took his side to King Power Stadium, in May 2015.
"It's a little bit strange, because last season they play against relegation and now they are playing for winning the title and that's football," Koeman told Southampton's official YouTube channel. "It's amazing what they did until so far and all the credits to Leicester.
"I've been very impressed. Of course, maybe in the beginning everybody was surprised because big teams were a little bit more struggling than usually, but they keep going and still they are the first on the table and very deserved, because they win, they are strong.
"Of course, they don't play European football, they don't have really big injuries and that makes it more easy for them, but again all the credits to them that they are still fighting to win the title. It's an amazing achievement for Leicester."
The sides drew 2-2 when they met at St Mary's in October, with a pair of Jamie Vardy goals in the second half seeing the Foxes claw their way back from two goals down.
"We had a good game against them," said Koeman. "The first 60 minutes we were the dominant team, 2-0 up, some good chances to kill the game and then of course you know, and you see the whole season, they have a great spirit in the team, they have a very high work rate in the team and they came back and Vardy scored the goals and that's difficult.
"But we had a good feeling about that game, but we know away or at home it's always a little difference."