Novak Djokovic admits he is struggling on grass courts but believes he may have learned a vital lesson ahead of Wimbledon.

The world No4 saved five match points to beat France's Florent Serra 5-7, 7-5, 6-1 and reach the last eight of the Halle tournament in Germany. The Serb saved three match points at 5-2 in the second set and then saved key points in the next couple of games.

Djokovic's battling victory brought back memories of his triumph over Spain's Guillermo Garcia-Lopez at Wimbledon in 2005, when the 22year-old saved six match points.

"It's a big lesson I learned here, to fight until the last point," said the former Australian Open champion. "My victory at Wimbledon meant a lot to me back then. Hopefully, this win can do something as well."

Djokovic faces Austrian Juergen Melzer tonight knowing that he needs to step up his game against the world No28.

He said: "I am not playing even close to my best on this surface. I am unbalanced, too far away from the baseline, too defensive. I need to get to the net more often."

Djokovic reached the semi-finals of Wimbledon in 2007 but, midway through his semi-final against Rafael Nadal, was forced to retire with back and foot problems.