Cody Gakpo was on target with his third goal of the tournament - making him the tournament's joint-leading marksman - while Donyell Malen struck twice from the bench to set up a last-eight clash with Turkiye, which beat Austria 2-1 later in the day.

Netherlands stumbled through to the knockout stages as one of the best third-placed teams, after a disappointing 3-2 defeat against the Austrians in its final Group D outing.

However, the Oranje were much improved as they ousted Group E winner Romania, and Koeman insisted his players must maintain these performance levels heading into the business end of the tournament.

"It is sometimes difficult to say why you play badly in one game and then can reach a high level the next match," he told reporters during his post-match press conference.

"We were sharp from the beginning today, and that made a difference. Maybe, one critical point is that it took too long to score the second goal.

"The result is always the most important thing, but we are Dutch and in the Netherlands, we have to play well and offensively. We know that.

"The whole performance today was outstanding, and that is what we need to do to continue in this tournament. This is the level. If we go down from this, we won't reach the final."

Koeman was part of the triumphant Dutch side at the 1988 European Championship, which was also held in Germany.

After guiding his side into its first quarter-final in this competition since 2008, he maintained his players must match the achievements of 36 years ago if it is to be mentioned in the same breath.

"We have great talent in this team," he said. "But, of course, you need to win something, then people can start to make comparisons with 1988."