The Holland midfielder scored in Saturday's derby win against Everton.
“It’s not just this season, he’s been playing like that from the moment last season started,” Van Dijk told liverpoolfc.com.
“He’s unbelievable. He’s very important to the way we play. For me personally, you see the amount of times I try to look for him. It benefits him, me and the team.
“He’s in incredible shape. He’s in the form of his life. He has to keep going. He’s still young. The expectation level will always be right up there and that’s what he has to try to reach every three or four days. It’s a nice challenge.
“During his first season here he hardly played. It’s a mix of everything: getting confidence, progressing and improving as a player, playing games at the highest level and knowing your role in the team.
“I’m very pleased for him because he puts in a lot of hard work to do what he is doing.”