By Nick Mulvenney

RIO DE JANEIRO (Reuters) - American Michelle Carter saved the best throw of her career for her final attempt to snatch the Olympic shot put gold out of the hands of Valerie Adams and shatter the New Zealander's dreams of a third successive title on Friday.

Adams, who would have become the first woman to win three straight Olympic titles in an individual athletics event, looked to have one hand on the medal with the only throw over 20 meters and only Carter and herself left to go in the final round.

Carter stepped into the ring for the sixth and final time, though, and launched the shot further than any American woman had before to take the lead with a heave of 20.63 meters.

Knowing Adams was still to throw, Carter would only allowed herself a discreet double thumbs-up in celebration and she watched on nervously as Adams managed only 20.39 with the final throw of the tournament.

Adams's second throw earned her silver, the same medal she received initially in London four years ago before Belarusian Nadzeya Ostapchuk was stripped of the title for a doping offense.

Carter, who added the Olympic gold to the world indoor title she won in Portland this year, looked close to tears as she was hugged by her fellow finalists with Adams among the first to embrace her.

Anita Marton won the bronze with her final throw of 19.87, a Hungarian national record.

Adams has dominated her event for much of the last decade and in Moscow in 2013 became the first woman to win four consecutive individual titles at the world championships.

Knee, elbow and shoulder operations in the last 12 months cost her the chance to go for five in Beijing last year and she only slowly moved back toward her once dominant form again this year.

World champion Christina Schwanitz of Germany had only two legal throws in the final round and finished sixth.

(Editing by Ed Osmond)