EUGENE, Ore. — Anyone who saw the way he slowly uncoiled from his crouch and lumbered down the track during his first race at nationals could tell — something was wrong with Tyson Gay.

It turned out to be something big — a hip injury that forced one of the world’s top sprinters and one of the top challengers to Usain Bolt out of the meet, off the U.S. team for worlds and into rehab mode with the London Olympics only 13 months away.

Gay said he made the decision Friday as he was talking to his agent’s assistant while trying, without much success, to warm up before the 100-meter semifinals.

“He pulled me to the side and said, ‘I can’t take it, seeing your face keep grimacing,’” Gay told The Associated Press. “He said, ‘I know you do it because you love it and it’s all you know.’ I decided to try to get ready and take care of it.”

The hip has been bothering Gay for most of the year, and after a lackluster run in Thursday’s prelims — a major effort in a race he can usually coast through — he said it was, in fact, bothering him.

Instead of fighting through it Friday, where he needed a top-three finish to secure a spot in worlds, he chose to shut it down. He won’t run in the 200, either. Afterward, as he hobbled around the perimeter of Hayward Field, he was downcast, and not only because this all but denies him a chance to challenge Bolt this year.

“I’m real bummed about not running, period,” Gay said. “It’s all I know, it’s all I love. It’s what I study every day.”

Gay said he couldn’t look too far into the future. Because he competed in nationals, he is technically eligible for the U.S. relay pool at worlds, though if the injury turns out to be significant, it’s unlikely he’d receive one of the two spots that don’t go to the 100 qualifiers and the alternate.

It means the U.S. team will probably have to do without its top sprinter of the last several years — the 2007 world champion in the 100 and 200 and the American record holder with a time of 9.69 seconds. And it would be a significant setback in Gay’s quest to catch Bolt, who holds the Olympic title and world records in the 100 and 200.

The men’s 100 prelims at the London Olympics are Aug. 4, 2012, a date very much on Gay’s mind when he made the decision to end his stay this week in Eugene.

“I’ve been bandaging it up, couldn’t really take any more,” Gay said. “I decided to stay healthy. It’s pretty painful running in pain.”

Gay’s absence put a different twist on the final, where Walter Dix, Justin Gatlin and Michael Rodgers finished 1-2-3 to earn the three spots at worlds. Rodgers, the 2009 national champ, sneaked into a spot that probably would’ve gone to Gay had he been healthy. Gatlin, however, was the bigger news. Back at nationals after serving a four-year doping ban, the 29-year-old showed he’s still one of the best in the world. The 2004 Olympic champion was crying when he crossed the finish line.

“I think that before the finish line I just let it all out with a roar,” Gatlin said.

Carmelita Jeter won the women’s 100 in 10.74 seconds. Marshevet Myers and Miki Barber earned the second and third spots at worlds.

Watching from afar will be Gay, who languished through his prelim Thursday in a wind-aided 10.01 seconds, finishing second to Ivory Williams. Williams came off the track excited after the win, but on Friday, he said he learned all he needed to know while watching a replay of the race.

“I could tell he was running unbalanced, so I knew he was hurt,” Williams said.

Earlier this month, Gay’s injury contributed to a lackluster performance in New York, when he lost a rain-soaked race to Steve Mullings of Jamaica. Only a few weeks earlier, however, he showed how good he can be when he ran 9.79 at a meet in Florida — the world’s best time this year.

It wasn’t the first time he’s posted those kind of marks.

Heading into 2008, Gay was the defending world champion and looked like a good candidate to hang onto the title of “World’s Fastest Man.” But soon after the year began, Bolt burst onto the scene, putting up startlingly good times in Jamaica, then breaking Asafa Powell’s world record by running 9.72 and routing Gay by 0.13 at a race at Icahn Stadium in New York.

Gay came into Olympic trials that year still considered among Bolt’s top challengers, but he injured his hamstring during 200 prelims and went pinwheeling down to the ground in the first turn.

He wasn’t the same in Beijing and failed to make the final, which made him a spectator when Bolt set the record again, this time at 9.69 seconds while showboating to the finish. Bolt pushed the mark down to 9.58 at 2009 worlds, still more than 0.1 better than Gay’s best time.

For the near future, Gay will be relegated to scouting Bolt and the rest of the competition instead of racing against them.

“I’ll probably go home today and watch film,” he said. “This is what I do. I enjoy watching the races. I just love it. I’m just bummed, period.”