The midfielder fears that his side are heading for Bundesliga relegation if they fail to raise their game when they take on Greuther Furth in the second leg of their play-off
Facing their first-ever demotion from the top flight, the German giants were nervous against Greuther Furth on Thursday at the Imtech Arena.
The second-division outfit started the game well and had a number of chances to open the scoring, with Niko Giesselmann wasting the best opportunity of the game early in the second half.
Hamburg eventually took control of proceedings but failed to breach Wolfgang Hesl's goal and Van der Vaart hopes that they can be more clinical in the return leg.
"They started very well and we came into the game late but we were better in the second half," the Netherlands midfielder said to the press.
"We still have a good chance of winning and we know we need to improve for Sunday. We're staying positive, though. A club as great as this belongs in the Bundesliga."
Hamburg will need to end an 11-game winless run away from home in all competitions at the Trolli Arena if they are to preserve their top-flight status.