SAN FRANCISCO -- To win one U.S. Open would suffice for most pro golfers, and winning at least two is a feat accomplished by just 18 players in the history of the championship.
Winning them back to back? There might not be a friendlier wager to make against such an occurrence.
Jack Nicklaus won consecutive Masters, but never two U.S. Opens in a row. Arnold Palmer won consecutive British Opens but managed just one U.S. Open title. Tiger Woods has won the British Open and PGA Championship (twice) in consecutive years but has failed to follow his three U.S. Open wins with another a year later.
What does that say about Rory McIlroy's chances this week at Olympic Club?
The 23-year-old set numerous records a year ago at Congressional Country Club, where he won the U.S. Open by eight shots. But a year later, that means little as he attempts to become the first player since Curtis Strange in 1989 and the seventh overall to repeat.
"It's the toughest test that we face all year and I'm not sure why there hasn't been a repeat champion, but obviously I'm going to try my hardest to make that happen this year," McIlroy said.
Repeating at the other majors is not easy, either, although it's happened more recently at each and more frequently than all but the Masters, which has seen just three players go back to back: Nicklaus (1965-66), Nick Faldo (1989-90) and Woods (2001-02).
The British Open has seen it happen 17 times, the latest Padraig Harrington (2007-08), preceded by Woods (2005-06).
Woods has repeated twice at the PGA Championship (1999-2000, 2006-07), but he is the only player to do so since the tournament went to stroke play in 1958. Prior to that, it had occurred six times.
"It's got to be something to do with the severity of the test," said 2010 U.S. Open champion Graeme McDowell, who tied for 14th last year in his defense at Congressional. "I think maybe the U.S. Open more than most majors requires just that little bit of luck. Right side of the golf course, right side of the weather.
"You've got to be 100 percent on with your short game. Everything working. Maybe a place like Augusta you can get away with a few errant shots. Augusta gives you opportunities to make up for mistakes. At the Open Championship, links golf, firm and fast, there [are] always chances. The U.S. Open always seems to be the more severe test."
Ernie Els has two U.S. Open titles but never came close to winning the next year. After a victory at Oakmont in 1994, he missed the cut the following year at Shinnecock. His victory at Congressional in 1997 was followed by a tie for 49th at Olympic.
"I remember both times I wasn't quite on my game," Els said. "It depends; you have to catch two golf courses you really like. That's the deal. Olympic is West Coast on the side of a hill. Congressional is on the East Coast, different. It's a difficult thing.
"I don't think it's pressure or anything like that. It's the golf course."
Not only is it difficult to repeat, it is not even easy to follow up with a contending performance.
Since Woods' victory in 2000, the best any winner has fared the following year is a tie for sixth. That was Woods in 2009 at Bethpage the year following winning at Torrey Pines. That is the only top-10 performance of the past 11 U.S. Open winners.
When Strange was on his way to winning the U.S. Open at Oak Hill in 1989 -- after defeating Faldo the year prior at The Country Club -- he had no idea so few had accomplished back-to-back victories in the championship.
The last time someone had done it was Ben Hogan in 1950-51.
"When you're in the middle of it, you don't give it much thought," said Strange, who will work the U.S. Open as an analyst for ESPN. "Nobody ever gave anybody much credit or chances to repeat because it just hasn't been done in so long. The first time somebody gave me a chance was after I shot 64 on Friday.
"On Saturday morning, I pick up the paper and now I hear it's been so long since Ben Hogan. I didn't have a clue who the last guy was. So now I know. But it doesn't make much difference because I'm in the middle of the tournament."
Strange went on to win his second straight Open, aided by a faltering Tom Kite on the final day. Strange would actually make a solid run at a third straight Open a year later at Medinah, where he entered the final round just two strokes off the lead before fading.
Only one player, Scotland's Willie Anderson, managed three straight Open victories, and that was in 1903-05. Those who followed him with consecutive wins were John McDermott (1911-12), the first American to win the U.S. Open, Bobby Jones (1929-30), Ralph Guldahl (1937-38), Hogan and Strange.
"I don't root against anybody," Strange said. "But the longer it goes and the farther removed we get from '89, the more proud I get of it.
"It's not so much winning back to back. It's more astounding that Nicklaus didn't do it or Palmer didn't do it or [Lee] Trevino didn't do it or [Tom] Watson didn't do it. You know, the truly greats of the game."
All four of those greats Strange mentioned did win other majors in consecutive years. Just not the U.S. Open.
And that is the history McIlroy faces at Olympic Club.
Bob Harig covers golf for ESPN.com. He can be reached at BobHarig@gmail.com.