Djokovic ncould not continue beyond the first set on Friday night
World number two Novak Djokovic has withdrawn from next week's Madrid Open after retiring from a match at the Serbian Open with breathing problems.
The 22-year-old Serb said allergies were to blame when he pulled out after losing the first set 6-4 to compatriot Filip Krajinovic on Friday.
Belgrade has been experiencing "a lot of pollen in the air", according to the ATP, governing body of men's tennis.
Djokovic lost an epic semi-final to Rafael Nadal in Madrid 12 months ago.
His absence this year means Britain's Andy Murray becomes the third seed, with the Scot drawn to face Uruguayan Pablo Cuevas or a qualifier in the second round having received a first-round bye.
Should Murray make it as far as the semi-finals he could face world number one Roger Federer, who will play either Spain's Carlos Moya or German Benjamin Becker in his opening match.
Home favourite and last year's runner-up Rafael Nadal will meet either Italian Andreas Seppi or a qualifier in the second round.
There was an early shock in the women's event on Sunday as French Open champion Svetlana Kuznetsova lost 6-3 2-6 6-0 to Shahar Peer in the first round.
The Russian has now won just a single clay match against three defeats this year.
Kuznetsova's compatriot Elena Dementieva crushed Canadian Aleksandra Wozniak 6-0 6-1, while Vera Zvonareva beat Melanie Oudin in a 6-3 6-4.
Justine Henin and Maria Sharapova are among the players in action on Sunday.