BELGRADE, Serbia (AP)—Top-seeded and defending champion Novak Djokovic retired with breathing problems during his quarterfinal match against Filip Krajinovic at the Serbian Open on Friday.
The second-ranked Djokovic said allergies were to blame for his respiratory troubles.
He pulled out after losing the first set 6-4 to Krajinovic, an 18-year-old fellow Serb who reached his first ATP semifinals.
Krajinovic will play third-seeded Sam Querrey, who defeated Igor Andreev of Russia 6-3, 2-6, 6-4. Second-seeded John Isner rallied to beat Richard Gasquet of France 2-6, 7-6 (6), 6-3, putting two Americans in the semis.
Gasquet broke twice to win the first set. But Isner saved three break points in the second, forcing a tiebreaker which Gasquet lost after a double fault. Gasquet opened the third set with a break, but Isner broke back twice for a victory and his first singles semifinals on clay this year.
Fourth-seeded Stanislas Wawrinka saved two set points in struggling past sixth-seeded Viktor Troicki of Serbia 7-5, 6-7 (3), 7-6 (6). Troicki wasted two match points at 6-4 in the third-set tiebreaker. Wawrinka then made good on his own match point to finish.
Wawrinka will meet Isner next.