South Africa's Oscar Pistorius made history by becoming the first amputee sprinter to compete at the Olympics.
The four-time Paralympic champion, 25, whose legs were amputated below the knee as a baby, finished second in his 400m heat in a time of 45.44 seconds to reach Sunday's semi-final.
"I didn't know if I should cry or be happy. It was such a mix of emotions," Pistorius told BBC Sport.
Defending champion LaShawn Merritt of the United States pulled up injured.
The 26-year-old, who failed a drugs test in 2009, appeared to be still suffering with an Achilles tendon problem that affected him earlier in the season.
He stopped running after 250m and crossed the line at walking pace.
"It's Olympic year - I tried to go for it, and it just didn't happen," he said.

Analysis

Image of Steve CramSteve CramBBC athletics commentator
"It pretty much went to script and, mark my words, that was a good run from Pistorius. It's been a tough road to qualify for the Olympic Games for Oscar. He ran a qualifying time last year but the South African Federation wanted him to repeat the qualifying time this year and now he has had his chance to represent his country at the Olympics."
Pistorius was impressive in his heat, coasting over the line behind Luguelin Santos of the Dominican Republic, who won in 45.04.
"Thank you so much to everyone who has supported me. Thank you so much to my family - I saw my gran in the crowd. It was the most amazing experience.
"This crowd is amazing. I've run so many times in the UK and it feels like a second home to me.
"I've got to thank my team, they trust me, I trust them. We've been together for nine years."
Pistorius ran in the individual 400m at the 2011 World Championships but did not progress beyond the semi-finals.
He also ran in the relay but, after running in the heats, he was omitted from the team which raced in the final as South Africa went on to claim silver.
Pistorius is also due to run in the 4x400m relay at London 2012.
British trio Conrad Williams, Nigel Devine and Martyn Rooney are also safely through to the semi-finals.