Even though his team recorded their third triumph in as many games, Van Marwijk was furious with his side's under-par display in their 2-1 win over Cameroon on Thursday.
"We could have easily lost that game and that was surprising to me," said Van Marwijk. "I spoke to my players very clearly after the Cameroon game.
- Netherlands v Slovakia: LIVE
"We played less well when we are ahead and we seem to think that it's going to be easy. "But in the last 16 you cannot afford that and that is what I said to them.
"We cannot underestimate any rival because otherwise we will be in for a surprise. You can always be surprised in football."
Slovakia coach Vladimir Weiss is hoping his team will approach the showdown with the same mindset they had against Italy on Thursday.
Having earned just one point from their opening two matches in South Africa, the Slovakians sparked their tournament into life with a 3-2 win over Italy in their final group game to progress to the knockout round.
"We came here to fight for something sensational," said Weiss. "But I don't consider our tournament so far as a miracle.
"We didn't start the tournament very well. We gave a goal away in the last seconds against New Zealand and our second match against Paraguay, which we lost, wasn't our game.
"But we played very well against Italy and we deserved to win that match. Miracles do happen and it's the round of 16 and everything is possible.
"The match will show something more about our level of performance and we will measure our forces against Holland."