Phil Mickelson took full responsibility for his error at the Presidents Cup as the United States saw its lead over the International team cut to one point.
Mickelson — playing in his 11th Presidents Cup — was hit with a penalty at the seventh hole in Incheon, South Korea, on Friday after he violated the "one-ball condition" during his fourballs match with Zach Johnson against Jason Day and Adam Scott.
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The ruling means that players cannot switch golf ball models during the round and is known as a one-hole adjustment.
As a result of his mistake the American was handed a penalty and as Johnson lost the hole it allowed their opponents to move ahead.
"It's not their [the officials'] responsibility for me to know the rules and if I had a question I should have asked," Mickelson told The Golf Channel. "We never had a non-one-ball rule that I can think of in these formats, and I don't know why I asked after I hit it. I just double checked.
"[But] it helped me get a little bit more focused because I was so angry at myself, that ended up with me making a birdie and eagle at 11 and 12 and got a little bit re-focused."
Despite the mistake, Mickelson and Johnson battled back to earn a half, but the 45-year-old rejected claims it felt like a win after rescuing their difficult situation.
"It doesn't feel like a victory," Mickelson said. "I made a mistake, I've never even heard of a match adjustment, that one's new."
The United States leads the International team 5.5 to 4.5 after the first two days of play at the Jack Nicklaus Golf Club Korea.