Kurt Felix, representing Grenada, didn't start the 110-meter hurdles at the beginning of the second day of competition at Olympic Stadium on Thursday.
The 2012 NCAA champion wrote on his Twitter account: "Bad. Hamstring in hurdle warmup couldn't start!''
Felix was 12th after the first day of competition (five events Wednesday and five Thursday).
Americans Ashton Eaton (world record holder) and Trey Hardee (world champion) are first and second, respectively, after eight of 10 events (javelin and 1,500 meters are remaining). They are attempting to become the first Americans to finish 1-2 since Milton Gray Campbell and Rafer Johnson did it in 1956.
The competition is expected to end around 2:30 p.m. MDT.