The Saints conceded 60 goals last season - only two teams outside the bottom three fared worse - as they finished just five points clear of the relegation zone.
Manager Mauricio Pochettino brought in Lovren from Olympique Lyonnais during the summer, and with the Croatia international alongside Fonte the Saints have conceded just twice in four Premier League matches.
"He's a great player first of all, and when you have a great player next to you it's easier to play," Fonte told the Daily Echo.
"I feel confident with him, we understand each other well. It's always going to get better, because we're still getting to know each other, but from the first moment I had a good understanding with him.
"I know more or less what he’s going to do in each situation, and he knows what I'm going to do, so at the moment we're in a very good place and I hope this partnership keeps developing and improving."