The United States hoped for a sweep but had to settle for only a silver medal in the men's shot put, the first of 47 track and field events to be contested. American Shalane Flanagan came through with a surprising bronze in the women's 10,000-meter run.
Gay pulled a hamstring while running in the 200-meter final of the American Olympic trials.
He won his first-round heat in 10.22 and then had a 10.09 while finishing second in the quarterfinal round. His chief opponents, however, all looked sharp. World record holder Usain Bolt of Jamaica ran a 9.92 in winning his quarterfinal heat and Asafa Powell, Jamaica's other gold medal hope, had a 10.02.
Richard Thompson of Trinidad and Tobago also ran a 9.9 while finishing in front of Gay.
The semifinals and final will be run Saturday.
Tomasz Majewski of Poland won shot put gold with a throw of 70-6 3/4. American Christian Cantwell took the silver and Andrei Mikhnevich of Belarus the bronze. Pre-Olympic favorite Reese Hoffa of the United States finished seventh and American Adam Nelson fouled on all three of his attempts in the final.
Flanagan, who did not make the finals of the 10,000 in Athens, finished third in the distance race behind Tirunesh Dibara of Ethiopia and Elvan Abeylegesse of Turkey.
American Hyleas Fountain had the lead through the first four events in the heptathlon. She was in front of Natalia Dobrynska of Ukraine by 64 points.