Watson and Simpson pummeled Scotland's Paul Lawrie and Swede Peter Hanson 5&4, then Phil Mickelson and Keegan Bradley beat Rory McIlroy and Graeme McDowell 2&1 to give the home team a 4-2 lead over holders Europe.
The Americans, who had split the opening session with Europe 2-2 after a cool and breezy morning, were also leading in one other match while trailing in the fourth.
Long-hitting Dustin Johnson and Matt Kuchar, who sat out the morning session, were two up on Englishman Justin Rose and Germany's Martin Kaymer after 13 holes while Steve Stricker and Tiger Woods were one down to Britain's Lee Westwood and Belgian rookie Nicolas Colsaerts after 14.
Held back by captain Davis Love III for the morning's foursomes, Watson and rookie Simpson came out all guns blazing in bright sunshine, piling up nine birdies between them before sealing their win at the par-five 14th.
The crowd crammed around the green erupted in loud cheers after Watson was conceded the team's 10th birdie after lagging his eagle putt close to the hole.
"We ran into a bit of a wall," Hanson said after the European duo's total of six birdies was swept aside.
"They were nine under through the first 10 holes, and we just couldn't jump on the train. That birdie train left, and we were six down after 10. It's hard to get back."
TOOK CONTROL
Mickelson and Bradley, who had won together in the morning, birdied the first three holes to take control against world number one McIlroy and fellow Northern Irishman McDowell.
Though the Europeans came from four down after eight holes to trail by two with two to play, Mickelson secured victory with a stunning tee shot to within two feet at the par-three 17th.
Mickelson and Bradley, who had outplayed his more experienced partner with a personal haul of six birdies, hugged each other in celebration as the crowd erupted with deafening cheers.
"It could be the best day of my life," said a jubilant Bradley, who has been mentored by Mickelson since he made his debut on the PGA Tour last year. "It was so much fun."
Bidding to win the Ryder Cup for only the second time in six editions, the Americans had trailed in all four of the morning's opening foursomes before fighting back to split the session.
Bradley and Mickelson earned the first point by beating Britain's Luke Donald and Spaniard Sergio Garcia 4&3, then a second when Zach Johnson and Jason Dufner defeated Westwood and Francesco Molinari of Italy 3&2.
McIlroy and McDowell came through for Europe when they edged Jim Furyk and rookie Brandt Snedeker one up after a fluctuating battle in the top match before Ian Poulter and Rose squared the ledger by beating an out of sorts Woods and Stricker 2&1.
Europe, under the captaincy of Jose Maria Olazabal, field one of their strongest ever lineups but face a challenging task to retain the trophy with the U.S. having lost only three times on home soil since the matches began in 1927.
(Editing by Julian Linden)


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