Former triple Olympic [Images] gold medallist Marion Jones [Images] ended her world championship bid without running a competitive step at the US trials in Carson, California on Friday.

Concerned about a hip flexor injury, Jones dramatically walked off the track after testing herself before the final preliminary race of the women's 100 metres.

"She is out of the meet, and will not try to run in the 200 metres," her agent, Charles Wells, said.

"She injured her hip flexor a week ago in a workout. We didn't think it was something that was very serious and we thought she was ready to run.

"But she got to the track and started to feel it, so she decided to pull out rather than risk more injury," Wells added.

"In two weeks she will be fine to run."

The US championships are serving as the American trials for August's world championships and Olympic 100 metres champion Justin Gatlin needed a referee's decision to keep alive his hopes of meeting Jamaican world record holder Asafa Powell [Images] there.

Gatlin, a heavy favourite, was reinstated in the men's 100 metres an hour after being disqualified for a false start.

The reinstatement came after officials reviewed a computer printout, which showed there was motion in the lane next to Gatlin, perhaps causing the Olympic champion to move, USA Track & Field spokeswoman Jill Geer said.

"It was a referee's decision," Geer added.

Gatlin now advances to Saturday's 100 metres semi-finals. The finals also are on Saturday.

"I reacted to another athlete in the field and I guess that [false start] was not called," Gatlin said. "I reacted to somebody else and that's all I can really say."

Olympic 200 metres champion Shawn Crawford clocked 10.10 seconds to lead first-round 100 metres qualifiers. Sydney Olympic gold medallist Maurice Greene was second-fastest at 10.12 seconds.

Jones's partner, former world record holder Tim Montgomery [Images], had withdrawn from the event earlier in the day.

Four-time world champion Allen Johnson later ran the fastest 110 metres in the world this year when the 34-year-old clocked 12.99 seconds to win the US title.

"The old man just won't go away," said third-placed Terrence Trammell.

Dominique Arnold was second in 13.01 seconds and Trammell ran 13.02 as Johnson predicted a US sweep in Helsinki.

Jones had run poorly this year and considered herself "a long shot" to make the world championships. But she needed to run faster than only two runners on Friday to advance to Saturday's second round.

She took one warm-up run down the straightaway for about 40 minutes, walked down the track a second time before returning to the starting area, picking up her clothing and leaving the track.

Montgomery pulled out of the championships "because he can't concentrate on running," Wells said.

"He is in great shape physically, but he can't concentrate because of what is going on off the track," Wells said.

Montgomery, who has fared poorly in races this year, is awaiting a decision by the Court of Arbritation for Sport (CAS) following a hearing earlier this month on U.S. Anti-Doping Agency (USADA) charges of serious violations in connection with the BALCO doping scandal.

He has never failed a doping test and is eligible to compete until a CAS decision, which is expected in late July.

"He is not retiring, but wants to wait until later this summer to run," Wells said.

Jones also has been under scrutiny by USADA but has never been charged with a doping offence. She also has never failed a drugs test.

In other competition on the second day of the four-day meeting, Olympic silver medallist Bryan Clay won the decathlon with 8,506 points and Olympic 400 metres gold medallist Jeremy Wariner topped the semi-finals of his event in 44.82 seconds.