Six singles matches were still to be played in the afternoon at Royal County Down.
GB&I went 3 up with four holes to play when Danny Willet's tee shot on the par-3 14th caromed off a hill and onto the green, striking the flag before it stopped 3 feet away for birdie, and GB&I remained 2 up when both teams made par on the 16th.
Jonathan Moore gave the Americans life with a 25-foot birdie putt on the 17th to pull within one hole, then Simpson earned an unlikely halve with his long eagle putt.
'It broke right at the beginning and left at the end,' Simpson said. 'It was such a cool thing. It looked good from about 10 feet out. I thought it was too hard at first.'
Each team of amateurs won a foursomes match.
Billy Horschel and Rickie Fowler took the lead on the third hole and beat Lloyd Saltman and Rhys Davis, 4 and 3, putting the first point on the board for the Americans. It looked as though the United States would win the second match, but U.S. Amateur champion Colt Knost and Dustin Johnson took bogey on the 18th hole to halve with Rory McIlroy and Jonathan Caldwell.
GB&I earned its only victory when John Parry and David Horsey defeated Trip Kuehne and Kyle Stanley, 2 and 1.
The Americans lead the Walker Cup, 32-7-1, but have not won overseas since 1991.
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