Serbian tennis star Viktor Troicki has been suspended for 18 months by the International Tennis Federation after violating doping regulations.
The federation said that Troicki -- a Davis Cup teammate of world No. 1 Novak Djokovic -- was guilty of "refusing or failing without compelling justification to submit to sample collection."
Troicki, ranked 53rd in the world, is one of the most high-profile players to violate doping rules in tennis.
The tennis federation said Troicki was notified on April 15 that he'd been selected to provide a urine sample and a blood sample.
The sport's governing body said he provided a urine sample but not a blood sample.
Troicki told an independent tribunal he'd been assured by antidoping officials the blood sample was not necessary because he was feeling unwell that day.
But the federation said no such assurances were given.
Based on reporting by Reuters and AP