The defending champion and top seed, Novak Djokovic was forced to retire with allergy problems and fatigue from the Serbia Open during his quarter-final with fellow Serb Filip Krajinovic in Belgrade.

Novak pulled out at the start of the second set after losing the first 6-4.

"It was obvious in the first match that I have problems to deal with it (allergies). It's just positive that I retired against Serbian player and not somebody else" - said Nole.

He added that his health problems are just temporary:

"My participation in Madrid is in doubt. I don't feel so good. That's what we will decide within the next 24 hours," he added.

"How much this problem will last, it depends on who much I will take rest. When you have this kind of problem, recovery requires rest, not physical effort. I will rethink about the next week, but the most important thing for me is to be able to play at Roland Garros and Wimbledon."

Krajinovic, who reached his first ATP semi-final, will now meet Sam Querrey, who defeated Igor Andreev of Russia 6-3, 2-6, 6-4.

Updated: Novak Djokovic retires in Belgrade with breathing problems