Indianapolis:Olympic swimming star Michael Phelps pulled out of the 100m freestyle at America's World Championship trials on Friday with a sore neck.
Phelps said he received treatment, but "when I hopped in the water and could literally barely turn my head to breathe, we thought it was better not to risk anything the next two weeks."
World Championship swimming competition starts in Rome on July 26, and Phelps had already booked three individual events - the 100m and 200m butterfly and the 200m freestyle.
All three were among his unprecedented eight-gold haul at the Beijing Olympics last August.
Phelps said he actually woke with a stiff neck on Thursday - from "sleeping wrong" - but swam through it to seize the 100m butterfly world record with a time of 50.22sec in the evening's final.
"It wasn't fun swimming the 100 fly with that," he said. "I could barely move my neck to breathe. I definitely didn't feel 100 per cent."
While much attention has been focused on his bid to revamp his freestyle to challenge the world's best in the 100m free, Phelps admitted the decision to withdraw was made easier by the knowledge that he wouldn't have swum the event in Rome.
"To be quite frank, Michael was not going to swim the 100m free in Rome because it conflicts with the 200m fly, and the 200m fly is going so well," Phelps's coach, Bob Bowman, said.
Phelps has indicated he feels he has unfinished business in the 200m butterfly. Although he won the event in world-record time in Beijing, he didn't hit the goal time he had set for himself.
He was clearly frustrated when he couldn't improve the record in the final here on Wednesday, a fact that fuels his ambitions for the event at the Worlds, where the 200m fly final and 100m free semi-finals are both scheduled on July 29.
"I'm disappointed not to be swimming (the 100 free) here," he said. But the 200 fly is a big race. That's a race I do not want to give up."
Phelps will almost certainly swim three relays in Rome, Bowman noting that anyone on the team can be selected for the 4x100m free relay, regardless of whether he swam it at trials.
Phelps posted the American record of 47.51 in leading off the US relay that captured gold in world record time in Beijing.
"We'll work it out," Bowman, who is also the head coach of the US men's team, said of the relay selection. "I don't think there's any question that Michael has proved he can swim fast on a relay."
Phelps said he planned to take it easy on Friday, then concentrate on preparing for Rome.
"I've got to get back in the water and train for the next two weeks and hopefully go faster than I did here."
Both Phelps and Bowman said there were technical improvements Phelps could make to improve on his performances here.