The toughest test in golf once seemed so easy to figure out.

The U.S. Open will be held at Torrey Pines, a public course that has become Tiger Woods' private domain. He won the Junior World Championship there as a teenager, and his record at the Buick Invitational on the PGA Tour is astounding. He has won six times, including the last four in a row, and has never finished out of the top 10 or more than four shots behind.

Woods shot a 67 on the South Course in his first round of the 2008 season, and a caddie standing behind the 18th green said, "He just won two tournaments with one round." Woods wound up at 19-under par and won by eight shots to set a tournament record.

"What he's going to do is screw the U.S. Open up for everyone else," Fred Couples said that day.

But that was before Woods had surgery on his left knee two days after the Masters, his second such surgery in five years. And that was before the world's No. 1 player sat out two months, leaving him without a competitive round until he tees it up Thursday on a Torrey Pines course set up for the U.S. Open, long known as golf's toughest test.

Now who's the favorite?

"That's a great question," Couples said. "In March, it was an easy one to answer."

So much has changed since Woods showed up at Torrey Pines in January to start his season. He said the Grand Slam was "easily within reason," and there was even talk of a perfect season.

It has become anything but that.

Two days after a runner-up finish at the Masters — so much for the Grand Slam — Woods had arthroscopic surgery on his left knee that cleaned out cartilage, among other things. Only those in his inner circle saw it coming. He said it had been bothering him since last year, and Woods used the offseason to strengthen his leg with hopes of avoiding surgery.

The recovery was supposed to be six weeks, in time for a trial run at the Memorial. But he said he wasn't ready, and there was a slight limp during his news conference that week to promote his AT&T National tournament.

"It's not like I haven't been down this road before," Woods said, alluding to his '03 victory in the Buick Invitational after missing two months with knee surgery. "I know what it takes to win a tournament after having a procedure done, and it's just a matter of being prepared, getting all my practice time in, making sure my shots are how I want them, trying to understand what my misses might be.

"But you don't really know until you get under tournament heat what your misses are going to be."

No one really knows anything.

Swing coach Hank Haney went to Florida when Woods started hitting balls a week before the Memorial. Woods said some practice sessions were long, others he cut short. With a week to go before the U.S. Open, he had not played an 18-hole round.

"He's progressing," Haney said. "But there's no way to predict progression. When I was there, he hit as good as I've ever seen him hit it. It's the stamina for the walking. He was hitting it great, drivers, everything. No problem at all. It's more of a walking issue."

It helps that the U.S. Open is held at Torrey Pines, a course he has been playing since a kid and knows his way around. It is different from Winged Foot, where Woods missed the cut in the 2006 U.S. Open — the only time he has done that in a major — after missing nine weeks to cope with his father's death.

What might not help is that it's the U.S. Open, the major Woods has won the fewest times (twice).