BERLIN — Dayron Robles hit the first hurdle that came his way Wednesday, a sign that one of the biggest stars of the world championships is struggling with an injury and might not make it into the men's 110-meter hurdles final.

Shortly after the Cuban pulled off his Che Guevara training jersey and got out of the blocks, his old hamstring injury acted up again. After his awful start, Robles never looked comfortable and needed to rely on his natural speed to squeeze into Thursday's semifinals.

The Olympic champion qualified — barely — as the third place finisher from his heat in 13.67 seconds, .76 off his world record time.

"It hurts, and I can't run as well as I usually can," Robles said, leaving him in doubt for the rest of the championships.

"I can't say that I am going to run, and I can't say that I am not going to run," Robles added. "I will work hard with my team, doctor and physio, to see what my chances are."

Quitting is a very tough option for Robles.

"I promised Fidel Castro to bring home the gold medal for him," Robles said of the former Cuban leader. "And I want to keep my promise."

The 110 hurdles was billed as the top event of the Beijing Olympics last year, but turned into a huge letdown when local favorite Liu Xiang of China was injured and didn't make it past the first round.

Liu still has not come back, and Robles' troubles are reminiscent.

The Cuban first injured his left hamstring at a meet in France in February and said he always had a nagging suspicion it was not fully recovered.

Two weeks ago, it started troubling him again.

"My muscles feel rigid and I cannot run at the top of my form," Robles said.

A less-than-fit Robles throws open the race for gold, especially after Dexter Faulk failed to get through the U.S. trials. Still, Robles's weakness opens the door for American hurdlers David Payne, who was second in Beijing, and Terrence Trammell.

It could help the U.S. team reinforce its standing on top of the medals table. And late Wednesday, the United States brings one of its greatest athletes to the track.

Defending 200-meter champion Allyson Felix will try to lift the gloom hanging over the American sprint team when she enters qualifying, freshly boosted by a double gold won by teammates Sanya Richards in the 400 and Kerron Clement in the 400 hurdles.

But much like the U.S. runners facing Usain Bolt in the men's 200 semifinals late Wednesday, Felix will have to battle the surging Jamaican women sprinters.

After being the big star with three golds at the last world championships in Osaka, Japan, Felix was upstaged in Beijing in the 200 by Veronica Campbell-Brown.

With the U.S. sprint team already down 2-0 to Jamaica in golds at the Olympic Stadium in Berlin, Felix will have to step up and bounce back from her Beijing defeat.

In the biggest upset of the morning session, two-time defending champion Franka Dietzsch of Germany failed to make the women's discus final.

The 41-year-old Dietzsch had two invalid throws and then landed a distance of 58.44 meters, good enough only for 23rd place in qualifying.

In the high jump, defending champion Donald Thomas of the Bahamas was eliminated with a jump of 2.27.

Another former champion, Tirunesh Dibaba, failed to make it to the start of the 5,000. Her no-show left Ethiopian teammate Meseret Defar as the favorite for Sunday's final.

The Dibaba family was still represented by Tirunesh's 18-year-old sister, Genzebe.