Zadar - Jack Sock is out of the United States team for the Davis Cup semi-finals against Croatia because of a hip injury and has been replaced by Ryan Harrison.
US captain Jim Courier says Sock was hurt last week while playing in the doubles semi-finals at the US Open with Mike Bryan. Harrison was already on standby for the Davis Cup.
Courier says "it's a factor in Davis Cup all the time, having to be prepared for these last-minute changes."
Sock is ranked No 17 in singles and No 2 in doubles.
The winner of the best-of-five series will meet either France or Spain in the Davis Cup final.