Novak Djokovic has retired down 6-4 to teenage compatriot Filip Krajinovic in Belgrade because of breathing problems apparently caused by allergies.
Djokovic, whose family owns the Belgrade tournament, was the defending champion and biggest attraction at the event.
The 18-year-old wildcard Krajinovic, who recorded his first ATP match earlier this week, now finds himsef in his first career semifinal, where he will play Sam Querrey.
Querrey became the second American semifinalist in Belgrade when he defeated Igor Andreev in the quarterfinals. Earlier in the day, John Isner won against Richard Gasquet. The two Davis Cup teammates have fared better in Belgrade this time than they did in March, when the American team lost to Serbia during the first round of Davis Cup.