Charlie Riedel, Associated Press
The US celebrates after a three-run home run by Crystl Bustos.

BEIJING — The three-time defending gold medalists have spent the better part of what could be the sport's last Olympic appearance trying to talk up their opposition and convince the world that while the U.S. may be dominant in the sport, the sport is still competitive.

That was not necessary in Wednesday's semifinals.

The U.S. had its toughest battle yet against a Japan team the U.S. defeated in the gold medal game in Sydney in 2000 and escaped in extra innings in Athens in 2004.

It went to extras once again Wednesday, before the U.S. broke it open with four runs in the top of the ninth, the game-winning hit delivered by Caitlin Lowe and the big blast a home run by Crystl Bustos, for a 4-1 victory to move into Thursday's gold medal game.

It was the American's 19th straight victory. They have now outscored their opponents 57-2 in this tournament and 108-3 over the last two Olympics.

Lowe opened the ninth-inning rally by driving in Natasha Watley with a slow single up the middle. Jessica Mendoza followed with a walk. Watley, the previous inning's last batter, had begun the inning on second by as dictated by international rules to speed up play.

Bustos, tied for the team lead with four runs in the tournament, launched a home run about 250 feet into the left field stands.

Cat Osterman relieved winning pitcher Monica Abbott with one out in the bottom of the ninth after Japan scored a run. Osterman retired the side to secure the win.

The U.S. had threatened a rally in the eighth, the first of extra innings, when pitch-hitter Lauren Berg led off with a pinch-hit single to put runners on first and third with no out.

But Japanese pitcher Yukiko Ueno struck Lovieanne Jung and Tairia Flowers, then got Watley to ground out to the pitcher to end the threat.

Held scoreless through the five innings, the longest it has gone without scoring in the tournament, the U.S. appeared to be mounting a rally in the top of the sixth when Flowers led off with a single and with one out, Lowe was hit by a pitch to put two runners on with one out.

But the next batter, leftfielder Mendoza, hit a shot to Japan third baseman Meg Hirose, who then fired to first to double off Lowe to end the inning.

Abbott was strong through six, fanning 10 and striking out the side in the bottom of the sixth. But she surrendered her first walk in the bottom of the seventh, then gave up a single up the middle to first baseman Rie Sato to put the game-winning run in scoring position with two out.

Abbott then got perhaps her biggest strikeout when she fanned Megu Hirose to force the game into extra innings.

Under the Olympic softball playoff system, the U.S. advances to the gold medal game while Japan still has a chance to play for gold if it can defeat the winner of the semifinal between Australia and Canada later Wednesday.

Japan, viewed as having the best chance to upset the U.S. going into the tournament, did not exactly demonstrate that in the teams' first meeting last week, a 7-0 loss in which the U.S. scored four times before the first out.

Japan came out considerably stronger Wednesday behind the pitcher of their ace Yukiko Ueno, who had struck out 18 in three previous wins with an earned run average of 1.11.

Ueno retired the first six American batters and allowed just two hits through four innings. Lowe led off the top of the fourth with a single down the leftfield line but was thrown out trying to stretch it to a double and Ueno struck out Mendoza and Bustos, the No. 3 and 4 hitters, to end the inning.

The U.S. managed one more hit in the fifth, a single by Andrea Duran, who reached second on a fielders choice. But Duran and her team were stranded again when Ueno struck out Jung to end the inning.

The U.S. had come into the eight-team tournament in the strange position of having to defend their dominance, one theory as to why the International Olympic Committee voted to eliminate softball, along with baseball, as an Olympic sport in 2012 and maybe beyond.

In Athens, the Americans set a new standard for dominance, outscoring its opponents 55-1 and setting 18 new Olympic records en route to the gold medal.

The team in Beijing was looking every bit as dominant headed into the semifinals with Japan, setting a new home run record with 13 in seven games and outscoring its opponents 53-1 while extending its winning streak to 19 consecutive games.