LONDON, Aug 4 (Reuters) - LaShawn Merritt's defence of his Olympic 400m crown lasted only a few seconds on Saturday as the American pulled up injured in his qualifying heat.

Merritt, the fastest in the world this year with 44.12, appeared on track with a heavily strapped left thigh in a bid to protect a hamstring injury suffered when he pulled up with cramp running in Monaco on July 30.

It was obvious from his first few steps that he was not going at full pace and after rounding the opening bend adrift of the field he stopped running and trudged the remainder of the race looking a dejected figure.

“I think I need some more rest,” Merritt told reporters.

“I've been feeling a lot better, I feel like the strength was there, but when I went for it, I could feel it.

”I thought I could get through the round not 100 percent. I got out and got to the curve and I could feel it.“

Merritt, 26, served a 21-month suspension after a positive doping test for using of an over-the-counter male enhancement product in 2010 that contained the banned substance dehydroepiandrosterone (DHEA) and pregnenolone.

There was no intention to dope and nor to gain a competitive advantage, an international panel found.

He was cleared to run in the Olympics when the Court of Arbitration for Sport ruled last year on an International Olympic Committee rule that banned from the next Olympics any athlete sanctioned for six months or more.

Merritt has the year's two fastest 400 metres times with a best of 44.12 seconds.

He said he would probably not have run had it not been the Olympics and had yet to decide whether he would compete in the 4x400m relay.