A new men's 400-meter Olympic champion will be crowned at the London Games, after LaShawn Merritt of the United States pulled up lame in Saturday's heats for the event.
The favorites in the men's 100-meters did not have the same problem, as Jamaicans Usain Bolt and Yohan Blake qualified for Sunday's semifinals, as did Tyson Gay, Justin Gatlin and Ryan Bailey of the U.S.
Bolt, of course, captured the world's attention with his gold medal run in the 100 at the 2008 Beijing Games. He also won the 200m and helped Jamaica take gold in the 400m relay.
While everything went according to plan in the men's 100 heats in the day session at Olympic Stadium, there was no shortage of action in heats for the men's 400 on Saturday morning. In addition to Merritt's injury, double amputee Oscar Pistorius also moved onto the semifinals.
Merritt failed to finish his first race in qualifying when a left hamstring injury forced him to pull up midway through. The 26-year-old Merritt also helped the United States win a 1,600-meter relay gold in Beijing, but his status for that race here in London is now in question. The relay is scheduled for Friday.
"I have some more time before the relay," said Merritt. "If I still feel it (the injury to his left hamstring), I will let someone else run."
Meanwhile, South Africa's Pistorius became the first man to compete in both the Summer and Paralympic Games and he qualified for the semifinals.
Pistorius, who had both his legs amputated halfway between the knees and ankles at 11 months old, ran in the first of seven heats at Olympic Stadium. Nicknamed the "Fastest Man on No Legs," Pistorius runs with the help of carbon fiber artificial limbs.
The 25-year-old finished second in the heat with a season-best time of 45.44 seconds. Pistorius posted the 16th-best split of Saturday's heats.
"I didn't know whether to cry," Pistorius said. "I had a mixture of emotions. It was the most amazing experience, the crowd was amazing."
Belgium's Jonathan Borlee logged the fastest time in heats with a run of 44.43 seconds. Tony McQuay and Bryshon Nellum qualified for the semifinals for the U.S.
While Bolt coasted to a win in the fourth of seven heats Saturday, his time of 10.09 seconds tied Canada's Justyn Warner for ninth-fastest. Blake, who beat Bolt at the Jamaican Olympic trials, ran the race in 10.00 seconds flat to finish with the third-fastest time.
"I expected it, I'm running well," said Bolt of his comfortable sprint.
Bailey posted the best time with a run of 9.88 seconds and Gatlin, the gold medalist eight years ago in Athens, was next, .09 seconds behind. Gatlin is back at the Olympics after missing Beijing due to a four-year doping ban.
Gay finished eighth at 10.08 seconds.
Six of the seven events in the heptathlon have been contested and Jessica Ennis of Great Britain is still in the lead.
Ennis has been in first place after five of the six disciplines and, with 5,971 points, is 188 ahead of Austra Skujyte of Lithuania.
Ennis could become the first British athlete to win the heptathlon since Denise Lewis claimed gold at the 2000 Sydney Games. The champion will be determined tonight after the running of the final event, the 800-meter race.
American Hyleas Fountain, silver medalist in the heptathlon in Beijing, is currently ranked 27th. Sharon Day of the U.S. is 17th, while Canada's Jessica Zelinka is sitting in eighth place.
Defending heptathlon champion Natallia Dobrynska is out of the competition. The Ukrainian had a poor showing in this morning's long jump and chose not to compete in the heptahlon's sixth event -- the javelin.
In qualifications for the women's pole vault on Saturday, Russia's Yelena Isinbayeva, the two-time defending gold medalist, cleared 4.55 meters to move on in the competition. Americans Jennifer Suhr and Becky Holliday also qualified for Monday's final round.