* Woods secures decisive point for U.S.
* Americans win Cup for an eighth time
* Hold off spirited International rally in singles (Adds detail, quotes)
By Mark Lamport-Stokes
DUBLIN, Ohio, Oct 6 (Reuters) - The United States won the Presidents Cup for the eighth time in 10 editions on Sunday, beating the Internationals 18-1/2 to 15-1/2 after Tiger Woods clinched the victory by beating South African Richard Sterne.
With the Americans needing at least 18 points to retain the trophy at Muirfield Village Golf Club, Woods secured the decisive point for the third time in a row in the biennial team competition, safely parring the final hole of his singles match.
The world number one, despite suffering from back pain over his last five holes, came from one down after eight to seal a one-up win that gave the U.S. an unassailable lead with two singles matches still to finish.
"It feels good. It was a team effort this whole week," said Woods, who won four of his five matches, the best on either team. "We really played well, and gave ourselves a really nice lead going into the singles.
"And it was a tough day, tough conditions, rain, wind. It was tough all around. It was a long week but it was worth it," he added, referring to the lengthy weather delays over the first three days which led to a backlog of matches. "We did it."
The U.S. had needed only four points from Sunday's 12 singles encounters to keep the Cup, having established a lead of 14-8 earlier in the day after dominating the rain-delayed final foursomes session.
However, they had to work a lot harder than expected as the Internationals mounted a gutsy fightback, winning seven-and-a-half of the matches on a saturated layout amid sporadic rain showers and occasional sunshine.
The International fightback gave the Americans some brief moments of anxiety but they managed to regain control when it mattered and avoid a repeat of last year's final-day collapse at the Ryder Cup.
"These guys gave their hearts and souls this week," Internationals captain Nick Price said of his team.
The U.S. team bristled with six players ranked in the world's top 10 while the Internationals, whose only Presidents Cup victory came in 1998, had eight players from outside the top 25.
"We had a tall order this afternoon, nine-and-a-half points (to win) against a team of the stature and ability of the Americans," said Price.
"We were all a little down in the room this morning after the close, after the finish of the team matches. Today the guys just showed the grit and determination and the resilience that they have."
Looking for a fast start from the top of their singles lineup, the Americans secured their first win of the afternoon when Hunter Mahan beat Japan's Hideki Matsuyama 3&2 in the opening encounter.
Australian Jason Day had won the first point of the singles, crushing Brandt Snedeker 6&4, but the Internationals were still six behind and the biggest obstacle facing the U.S. seemed to be the weather with heavy storms forecast for later in the day.
PGA Championship winner Jason Dufner never trailed before beating Zimbabwean Brendon de Jonge 4&3, then Canada's Graham DeLaet spectacularly holed out for birdie from a greenside bunker at the 18th to edge Jordan Spieth one-up.
Zach Johnson wrapped up a comfortable 4&2 win over South African Branden Grace to put the U.S. ahead 17-10 before Masters champion Adam Scott put another win on the board for the Internationals with a 2&1 win over Bill Haas.
South African Ernie Els scraped past Steve Stricker one-up and Australian Marc Leishman sank a 14-footer from the fringe to par the last and beat Matt Kuchar by the same margin.
The Americans were kept waiting even longer for that elusive 18th point when former Masters champion Charl Schwartzel of South Africa came from one down after 13 holes to beat Keegan Bradley 2&1.
However Woods, who had won three matches out of four in partnership with Kuchar earlier in the week, delivered for his team as he split the fairway off the tee on 18, then struck an accurate approach to 30 feet.
He narrowly missed his birdie putt but Sterne conceded the par to Woods and the overall victory to the U.S., before the last two pairings finished off.
"Three in a row and Tiger has the winning point all three times - that's pretty good," said U.S. captain Fred Couples, who plans to stand down from the role after leading the team to three successive victories.
"I've had All-Star teams every single time and we've gotten nice leads. Maybe they relax a little bit because they are all playing so well. It just turned out that Tiger ended up getting the point and playing actually 16, 17, 18 very, very well." (Reporting by Mark Lamport-Stokes; Editing by Julian Linden)