OSAKA, Japan — The vacant lane nine said it all.

It was to have been Bryan Clay's lane. It would have been his path to a quick 400 meters, one of his strongest events in the 10-event decathlon.

But it didn't happen.

For a second consecutive meet, the 27-year-old graduate of Castle High School and Azusa Pacific University wound up on the injured list rather than completing his full 10-event decathlon assignment.

A strained right quadriceps muscle suffered in the high jump at the 11th World Championships of Track and Field forced Clay out of the running after four events yesterday at Nagai Stadium. He had taken himself out of the U.S. National Championships at Indianapolis on an injury in June.

"It was just too painful," said Clay, who had 3,558 points yesterday. "It was going so well, too."

With five events to go in the decathlon, Jamaican Maurice Smith had the lead with 4,525 points. Dmitriy Karpov of Kazakhstan was next at 4,439, and Czech Olympic champion Roman Sebrle had 4,434.

Clay still can't figure out what exactly happened. He hurt his right quadriceps as he went to plant his foot for his second attempt at 6 feet, 6 3/4 inches in the high jump. He heard a pop and slumped under the bar, falling onto the mat. He limped off the track and didn't return.

"I think my heel may have slipped," he said. "At least I think that's what happened. I really don't know."

After the injury, Clay tried to get ready for the last event of the first night — the 400 meters.

A massage didn't alleviate the knot, neither did the acupuncture. After running to test the leg, then conferring with doctors, he decided it was best to pull out.

"I wanted to keep going," said Clay, the 2004 Olympic silver medalist. "But it just got tighter and tighter. Maybe I could've run the 400. But I would've run it in 52 seconds, and it would've been pointless."

He paused.

"I wanted to perform well here, too," he said.

This was Clay's fourth trip to the world championships — and the third time the designation DNF (did not finish) was posted next to his name. He also was forced out of the 2001 Worlds in Edmonton and the 2003 Worlds in Paris.

"I really feel it for Bryan, but that's the nature of the event," said USA teammate Tom Pappas, the 2003 World champion who has had an array of his own injuries over the years, including one that knocked him out of the 2004 Athens Olympics.

And Clay had been off to a fast start in defense of his 2005 world title.

Clay, wearing Japanese flags painted on each shoulder, temporary tributes to both his hosts and his Japanese-American heritage, won the first two events, the 100-meter dash and the long jump.

Right out of the blocks, he was close to his Helsinki 2005 pace.

He clocked a 10.44 in the 100 and opened a 42-point lead.

Next came the long jump and Clay did even better than he did in Helsinki, spanning 25-1 1/4, and his lead over Smith went to 78 points.

But Smith, a former Auburn University athlete, rallied in the shot put with a near career-best of 56-10 as Clay had a 50-10 3/4.

Clay had hoped to find a way to get through the first day, but then he thought about the looming hurdles.

"That would've been tough. I would've struggled," he said. "It was better calling it quits."

He might do that with the season, too, a difficult decision less than 12 months from the Beijing Olympics.

"We're going to discuss what we want to do," Clay said. "It's been a tough season. Do I shut it down and heal up or keep going? We'll see."

Elliot Denman and The Associated Press contributed to this report.