Mickelson was intent on challenging Woods, who had yet to tee off, at Muirfield Village Golf Club.

"I don't think it's anything serious but it just got worse. I couldn't grab the club and I couldn't swing," said Mickelson, who was 2 over through 11 holes today before returning to the clubhouse. "I hit a wedge on 2 and it got really aggravated."

Earlier this week Mickelson, second on the money list to Woods, played at Oakmont, site of the U.S. Open in two weeks. That is where he apparently injured his wrist, although the couldn't remember precisely when he first hurt it.

"I was hitting a lot of chip shots out of the rough at Oakmont and I think that kind of aggravated it. I hit some drivers today and it started hurting a little bit, but nothing I didn't think I could handle," he said. "When I hit a wedge on 2 (on Thursday), it shot up my arm and it was OK, I could kind of half-clubbed it around. The 5 iron on 4 really hurt. It just got worse."

Mickelson said he would ice his wrist, see a doctor and could possibly play as soon as next week's stop in Memphis. But getting ready for the Open takes precedence over that.

"The U.S. Open is more of what we're gearing up for, as much as I'd love to play here and as excited as I was to play here and get back into the swing of it, I couldn't swing," he said.

The pain got to be too much after he hit his approach to the green on the par-5 11th hole.

"The wedge shot on 11 out of the divot jarred it pretty good," Mickelson said. "I just didn't feel like I could hit a shot on 12."

Mickelson's withdrawal erased one of the major storylines at the Memorial -- Mickelson's challenge to Woods.

Woods is No. 1 by acclaim, not even close. But that doesn't stop everybody from trying to speculate about who could sneak up and steal his crown.

"I've had different guys make different runs," Woods said after yesterday's pro-am at Muirfield Village Golf Club. "It was (David) Duval and then probably Phil and Ernie (Els), Vijay (Singh), Goose (Retief Goosen) _ a combination of those guys throughout the years."

With Mickelson coming off a big win at The Players Championship three weeks ago, some were saying he was primed to challenge Woods at the Memorial and then in two weeks at the U.S. Open at Oakmont.

It seems that every time Mickelson gets a big win -- the Masters in 2004, the PGA Championship in 2005, the Masters again in 2006 -- it gets Woods' attention.

A year ago, for instance, Mickelson won at Augusta and was in position to take his third major in a row before hitting that hideous drive on the final hole that cost him the Open at Winged Foot.

Still grieving the loss of his father, Woods wasn't a factor there, but then turned around and won the last two majors, the British Open and PGA.

With Woods facing impending fatherhood and the changes that can bring, now could be the time for someone to finally challenge for the top spot in the world.

Woods doesn't seem to be conceding anything, though.

"As long as I'm up there in the conversation, I'm doing all right," Woods said. "I keep pushing myself pretty hard, regardless. I always try and get better. You can't stay stagnant in any sport. You've always got to keep getting better because you know the guys are training harder, they're finding their game, they're in the gym, you're getting better athletes playing the game. So you've got to keep getting better."

Even without Mickelson, the field mirrors that of a major. Nine of the top 10 money-winners are on hand, along with former Memorial winners Els, Jim Furyk, Singh, Tom Lehman and Paul Azinger.

Masters champion Zach Johnson, still on a victory lap after winning the Masters in April and the AT&T Classic two weeks ago, burns to remain in such elite company.