SAN DIEGO — Tiger Woods wasn't sure if his left knee would allow him to finish 72 holes of the U.S. Open. After yet another defining moment at Torrey Pines today, he was thrilled to get a chance at 18 more.

Down to his last stroke, Woods holed a 12-foot birdie putt that curled into the right side of the cup on the 18th hole to force a playoff tomorrow with Rocco Mediate, who could only watch on TV as Woods delivered another epic moment in an Open loaded with them.

Mediate, trying to become the oldest champion at age 45, two-putted for par from above the ridge for an even-par 71 to finish at 1-under 283, the first time since 2004 that someone broke par in a U.S. Open.

It looked like it might be good enough when Woods and Lee Westwood of England, both one shot behind, hit into the bunker on each side of the fairway on the 527-yard closing hole and had to lay up.

Westwood's 15-foot birdie putt never had a chance, losing speed and turning away. He shot 73.

From an unpredictable lie in the right rough, Woods gouged a wedge out to 12 feet right of the pin. He started to backpedal as the putt neared the hole, paused to make sure it was in, then curled and pumped both fists toward him with his head to the sky.

"Unbelievable. I knew he'd make it," Mediate said as he watched from a scoring room.

Woods shot 73 and will be in a playoff for the third time in a major, this one 18 holes of stroke play on Monday.

The 50,000 fans at Torrey Pines, who thought they had seen it all during a most remarkable week, now get a little bit more.

It will be the first playoff at the U.S. Open — the only major that goes 18 holes of overtime — since Retief Goosen defeated Mark Brooks at Southern Hills in 2001.

And it was made possible by Woods, among the greatest clutch putters of all time, making a putt that simply couldn't miss.

"A little wobbly down there," he said of the poa greens, a grass that gets bumpier in the afternoon sun. "I played probably 2½ holes outside right. Just take it back and make a pure stroke, because once it starts slowing down there ... you don't know what's going to happen. All I could control is my stroke."

The birdie concluded a week in which Woods played the first two rounds with Phil Mickelson, shot 30 on his back nine Friday to get into contention, took the 54-hole lead Saturday with two eagle putts totaling 100 feet, and wobbled on a knee that often turned a megawatt smile into a painful grimace.

The knee didn't seem to bother him as much Sunday — certainly not when he launched into the biggest celebration of the week.

"I took some things to kind of relieve that," Woods said of the soreness.

Adrenaline maybe?

"Uh, that helps, too," he said.

Mediate made only one bogey over the final 13 holes, seizing on his best — and perhaps only — chance to win a major. He grazed the edge of the cup on a 15-foot birdie try at the 17th and hit a wedge too strong on the 18th, both pars keeping Woods in the game.

Monday will not be the first time they have tussled.

Mediate played with a 23-year-old Woods in the final round of the Phoenix Open in 1999, where he led by six shots and held on to win by three. It was one of his five PGA Tour victories.

"Battle royale," Mediate said of what awaits. "The thing that is most amazing is the man I'm going to play tomorrow has won 13 of these. It's amazing how much it takes. I gave all I had today and I can't complain."