* U.S. dominate second day's play at Medinah
* Need 4-1/2 more points to regain trophy
* Poulter gives Europe hope with strong finish (Adds details, quotes)
MEDINAH, Illinois, Sept 29 (Reuters) - Though Tiger Woods lost for a third time this week, the United States tightened their grip on the 39th Ryder Cup by taking a commanding 10-6 lead over holders Europe at Medinah Country Club on Saturday.
After red-hot duo Phil Mickelson and Keegan Bradley inspired the Americans to win the morning's foursomes session 3-1, Bubba Watson and Webb Simpson led the way in the afternoon's fourballs which were split 2-2.
Europe had trailed in three of the fourballs but Sergio Garcia and Luke Donald held off a fightback by Woods and Steve Stricker to win one up before Rory McIlroy and Ian Poulter came from two down to beat Jason Dufner and Zach Johnson by the same margin.
Matchplay specialist Poulter finished with five consecutive birdies, the last of them a 15-foot putt in fading light to give the Europeans a slim hope of retaining the Cup heading into Sunday's concluding 12 singles encounters.
A strong European team had arrived at Medinah closely matched with the U.S. but with several of their top players not at their best, they face a daunting challenge on the last day.
The Americans famously pulled off the largest Ryder Cup comeback at Brookline in 1999 when they trailed 10-6 going into the final day and the Europeans would have to match that to keep the trophy.
"Those last two matches were massive," Europe captain Jose Maria Olazabal, who played that year at Brookline in the decisive match, said of his team's strong finish on Saturday.
"That gives us just a chance. It's been done before in the past. Tomorrow is going to be a big day. I believe that momentum will come our way and why not tomorrow?"
U.S. captain Davis Love III, who has rested all 12 of his players for at least one session at Medinah, was delighted with his team's position, with just a further four-and-a-half points needed to regain the trophy.
"Our guys all played real well today," he said. "We lipped out a bunch of putts, and we hung in there, but we haven't lost a segment yet. We're just going to try tomorrow to keep that string going."
Left-hander Watson and Cup rookie Simpson earned the U.S. their first point on a sunny afternoon when they beat England's Justin Rose and Italian Francesco Molinari 5&4 in front of highly energised crowds.
Watson sealed the win with a two-putt birdie at the par-five 14th, but U.S. Open champion Simpson paved the way to victory with a red-hot putting display as he piled up seven birdies.
"Webb just was a little anxious, wanted to make all the putts," the long-hitting Watson said. "I did the par-fives and he did the rest."
Dustin Johnson and Matt Kuchar added another point for the Americans when they held off a late charge by Belgian Cup rookie Nicolas Colsaerts and Scot Paul Lawrie to win one up.
With the match all square after 16 holes, Johnson effectively delivered the dagger blow when he sank a curling 20-footer to birdie the par-three 17th and trigger thunderous cheers from the crowd.
"It was probably the loudest roar I've ever heard," said Johnson. "In that situation, it was probably one of the best putts I've ever made."
Woods and Stricker, benched for the morning's foursomes after losing their first two matches, seemed down and out when trailing Spaniard Garcia and Englishman Donald by four.
However Woods, who had struggled badly with his game on the front nine, sparked after the turn with a run of five birdies in eight holes to take the match to the 18th as chants of "U-S-A, U-S-A" echoed across the course.
Stricker, one of the best putters in the game, lined up a seven-foot birdie putt at the last that would have tied the match but his attempt lipped out.
"I've played well the last two afternoons and didn't get a point. It's tough," said Woods.
Players from both teams thrilled the crowds in the late afternoon with a series of stunning approach shots and clutch putts but it was Englishman Poulter, flinty eyed as he repeatedly shook his fists in celebration, who gave a Europe a timely boost with his spectacular finish.
"He was incredible on the way in, all the credit needs to be put on this man," said Northern Irish world number one McIlroy. "When Poults gets that look in his eyes, especially the week of the Ryder Cup, it's really impressive.
"We have 12 great players on this team and we just need to go out there tomorrow, believe in ourselves and try as hard as we can." (Editing by Julian Linden)