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, 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 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 a n d L e e W e s t w o o d o f 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 p u t t n e a r e d t h e h o l e , paused to make sure it was in, then clenched 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 today.
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 2 1 /2 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 surgically repaired knee that often turned a megawatt smile into a painful grimace.
The knee, which sidelined him from the Masters until this week, didn’t seem to bother him as much Sunday — certainly not when he launched into the wildest celebration of the week.
‘‘I took some things to kind of relieve that,’’ Woods said of the soreness.
‘‘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.
Today 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.
Woods has never lost a major when he had at least a share of the 54-hole lead, a n d h e c a m e c l o s e t o throwing this one away on a series of questionable decisions and poor shots.
He tried to reach the 13th green in two from 291 yards when a birdie was not necessary, then pulled it into a hazard and walked off with bogey to slip one shot behind Mediate, who had birdied the 14th ahead of him.
Woods then laid up with an iron on the 14th, where the tees were moved up to make it play only 267 yards, hit a sand wedge 20 feet beyond the hole and made par. And two shots into the right rough on the 15th led to a bogey that put him one shot behind.
Standing over an all-ornothing putt on the last h o l e , W o o d s a g a i n delivered.
It was reminiscent of the 2000 PGA Championship at Valhalla, where he made a six-foot birdie putt that broke both ways to get into a playoff against Bob May, the critical piece on his way to four straight majors.
‘‘It feels very similar to what Valhalla felt like,’’ Woods said. ‘‘If I didn’t make that putt, I don’t get to continue to keep playing. At best, I gave myself a chance to win the tournament tomorrow. And that’s all I can ask for.’’