By Mark Lamport-Stokes

BETHESDA, Maryland (Reuters) - Widely regarded as the toughest major to win, the U.S. Open starting at Congressional Country Club on Thursday is also one of the most unpredictable of all time with regard to a likely champion.

Former world number one Tiger Woods has lost the firm grip he once enjoyed at the top and he is a conspicuous absentee this week while he continues to recover from his left knee and Achilles tendon injury.

"Probably 10, 15 years ago there were only 10 or 12 players who could win but now it's so spread out," German Kaymer told reporters while preparing for Thursday's opening round.

"Golf is in such a good position at the moment because it's so volatile you can get a different winner every week," said Englishman Westwood, who is twice a runner-up in majors and three-times a third-place finisher.

"It's part of the challenge of the game at the moment. If I could pick out a favorite, I'd be working for (bookmakers) Paddy Power or BetFred or something like that."

The list of potential champions on the 7,574-yard Blue Course at Congressional this week includes richly talented young guns, ice-cool veterans and U.S. Open specialists.

Kaymer, who won his first major at last year's PGA Championship, will certainly fancy his chances, as will the in-form American trio of Steve Stricker, Phil Mickelson and Matt Kuchar.


Masters champion Charl Schwartzel of South Africa now knows he can triumph at the highest level while Britain's Rory McIlroy and Americans Dustin Johnson and Nick Watney will hope to build on lessons learned after relinquishing 54-hole leads in the last three majors.

Britain's Graeme McDowell does not plan to surrender the U.S. Open crown he claimed last year at Pebble Beach without a fight while American Bubba Watson and Englishmen Paul Casey and Ian Poulter each have the self-belief needed to win a major.

"Every time you have Tiger in the field, you've got to think he's going to have a good chance," said the 22-year-old Northern Irishman. "To have one of the main contenders not here gives the rest of us a little bit more of a chance."

As ever at a U.S. Open where 'plodding' rather than 'birdie hunting' is the name of the game, the ability to minimize errors and stay patient on slick greens and tight fairways flanked by thick, graduated rough will be defining traits in the make-up of this week's champion.

"If you're hitting it long and straight, it favors you here," said Englishman Donald, who claimed his third PGA Tour victory at the WGC-Accenture Match Play Championship in February. "But it's the same every week.

"I think U.S. Opens are all about controlling your golf ball, hitting fairways and hitting greens, as well."

Other likely candidates include double U.S. Open winners Ernie Els and Retief Goosen of South Africa, 2007 champion Angel Cabrera of Argentina and South Korean K.J. Choi, an eight-times winner on the PGA Tour who is renowned for his ice-cool composure.

It could also be a week to remember for one of the younger guns, who have already made a significant splash at major level.

McIlroy has already established himself as one of the best players in the world while Australian Jason Day, 23, posted his best major finish at the Masters in April, closing with a 68 to tie for second place.

(Editing by Julian Linden)