Kaymer, who collected his second career major title last June at Pinehurst, and Watson, the big-hitting US star who won his second green jacket last April, each finished the 36-hole showdown of major winners on six-under par 136 at Bermuda's Port Royal layout.
But in a replay of the 18th hole that both parred in the final round, Kaymer birdied on a 10-foot putt similar to one on the same line he had faced a few minutes earlier from twice the distance while Watson missed a short birdie bid to hand Kaymer the crown.
"I was surprised he missed it," said Kaymer. "You don't want to win a golf tournament that way. Of course I would take it, but you don't really want the other guy missing a short putt."
Watson rues windy conditions
Watson said the windy conditions ruined what he saw as a strong final putt.
"There's no way that ball should go that way, unless it's forced by the wind. It was Kaymer’s time and not mine."
Kaymer knew, however, that Watson's putt was trickier than it appeared.
"I had a similar putt earlier in the regular round, just half an hour before we were standing there in the playoff, but his was a little bit longer and I knew that it was not an easy putt.
"My putt, it was quite an advantage that I had a similar one earlier, just a little bit longer, but pretty much the same line and I knew it didn't break as much as I thought. So that helped me a lot. It was a good stroke and so it went in."
Rory McIlroy, who won the British Open and PGA Championship to boost his major title total to four, fired a 75 to finish third on 144, one stroke ahead of Jim Furyk, who closed with a 73.
Furyk made the field after McIlroy's double major win opened a space and Australian Adam Scott, the leader in a points placement system to determine alternates, passed on playing due to a schedule conflict.
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