Flowers drove Hiroko Sakai's first pitch of the second inning easily beyond the center-field fence, giving Osterman all the offense she needed to guide the U.S. to its 22nd straight victory and second consecutive World Cup title.

Bustos' double extended the lead in the fifth inning, but only after she fought back from an 0-2 count to push it full. Her hard roller bounded all the way to the left-field fence, allowing Natasha Watley and Jessica Mendoza to score.

Japan, the Americans' top challenger in international softball, hit only two balls out of the infield and managed just three hits against Osterman, the left-hander who played at Texas and threw 14 2-3 shutout innings at the 2004 Athens Olympics.

Osterman (2-0) struck out 13, including 10 of the first 13 batters she faced. In the second, Emi Naito reached on a squib shot that spun its way about halfway to the pitching circle, but was called out when she was hit by Rie Sato's grounder.

Osterman then struck out the next seven batters before Naito grounded out to shortstop for the first out of the fifth inning. One out later, Megu Hirose lined a single to center field — the first hit to reach the grass. She was called out for leaving first base early on a pitch, but a freeze frame replay showed she was still on the bag when the ball was in Osterman's hand.

Osterman struck out the side in the third, fourth and sixth innings. The game ended on a bizarre play when runner Aki Uenishi was called out for failing to return to first base quickly enough when Osterman took the ball into the circle.

Umpire coordinator Julie Johnson said Uenishi violated the "look back" rule by stepping off the base when Osterman entered the circle. Johnson said Uenishi had to either attempt to advance toward second or immediately return to first.

Uenishi remained on the base for several moments while Japanese coach Haruka Saito sought an explanation from umpires. U.S. players lined up and waited near home plate to shake hands with the Japanese players.

The Americans didn't have much success early against Sakai (2-1), who threw four shutout innings against the U.S. in a Canada Cup game earlier this month.

Flowers, the MVP of the Canada Cup after she hit .650 in that tournament, got the offense going. The first baseman from UCLA paced the American offense at the World Cup, going 6-for-13 and leading the tournament with three home runs and 12 RBIs.

The U.S. has won all 16 of its games this year, also winning the title at the Canada Cup. Entering the Pan American Games, the U.S. has outscored opponents 134-4 this year.

The U.S. has beaten Japan — the team that's handed the Americans their last four losses — six times in its current winning streak. All four of Japan's recent wins came behind ace Yukiko Ueno, who didn't play at the World Cup.