With Netherlands already through to the second round and Cameroon out, it took Robin van Persie's 36th-minute flash of inspiration to get things going at Green Point Stadium.
Samuel Eto'o's second-half penalty levelled matters until substitute Klaas Jan Huntelaar's 83nd-minute winner continued the group winners' momentum heading into a clash with Slovakia.
"I felt today we were less good than our last two matches," Van Marwijk said. "We were dealing with an opponent already leaving the World Cup and it's unpredictable in the all-or-nothing way that they could play.
"We needed about ten minutes to get used to it but after that period in the first half we started passing better and creating opportunities.
"When we went 1-0 up I expected us to play professionally, finish the match and score more. It required concentration but in the second half we started too easily, it was sloppy, and then you're bound to end up having problems.
"I'd been quite relaxed on the bench in the first half but in the second I thought, 'Oh God, Cameroon might score and we might end up losing'. That happened, when it went to 1-1. All of a sudden we were shaken awake and we scored a goal to go 2-1 up and, if it had gone on longer, we might have scored one or two more."