The Dutch face the South Americans in the last four at Green Point Stadium in Cape Town on Tuesday as favourites to reach the final of the world's biggest football tournament.
Van Marwijk said there was always a risk of losing focus after a surprise win, especially against a big team like Brazil.
"I think the next game is a very dangerous game. This is the moment that I have already talked about for two years," he told a press briefing following a training session in Johannesburg.
"When you beat Brazil and you are in the last four of the World Cup then there is a next game coming and the next game is Uruguay. They are fighters and we have to be very focused."
Uruguay beat Ghana, the last African side in the World Cup, on Friday after a penalty shootout to reach their first semi-final in 40 years but will be without striker Luis Suarez, who was sent off for handling the ball on the line.
STAYING FOCUSED
Van Marwijk said losing Suarez would not significantly weaken Uruguay because they have good replacements.
The Dutch were also not thinking about a possible final and did not care at this stage who the opponents might be, should they beat Uruguay.
"I don't think that way. When you want to be world champions you have to win every game," he said, adding that looking too far ahead was risky.
"The message to the team is to keep concentrating day-by-day, match-by-match and that is what we are going to do."
Dutch striker Robin van Persie was taken to hospital for a scan on an arm injury, while defender Joris Mathijsen was also in hospital to check on a knee problem.
Both players did not train on Saturday, with the session only for those who did not play a full match against Brazil.
Van Marwijk said the injury to Van Persie, who played 85 minutes on Friday, was not serious and his form had improved.
"I think Robin played better against Brazil in the last game so I have a lot of confidence in Robin van Persie," he said.
Van Persie reacted angrily when he was replaced 10 minutes before the end of the 2-1 win against Slovakia in the first knockout round, remonstrating with the coach.
