By Mitch Phillips

RIO DE JANEIRO (Reuters) - Michelle Carter produced the throw of her life with her last attempt to deprive New Zealander Valerie Adams a piece of Olympic history on Friday as the American won the women's shot put to close out a memorable first day of athletics in Rio.

Carter heaved an American record 20.63 meters to snatch gold from Adams, who had been seeking to become the first woman to win three straight Olympic titles in an individual athletics event. She took silver with 20.42m.

Earlier in the day, Tirunesh Dibaba failed in her attempt at that elusive hat-trick when she finished third in an extraordinary 10,000 meters won by Ethiopian Almaz Ayana in a stunning world record time.

That leaves the door open for Shelly-Anne Fraser-Pryce to have a crack at the 'three-peat' in the women's 100 final on Saturday after the Jamaican qualified fastest for the semis as the only woman under 11 seconds.

Organizers will hope her presence, and the first appearance on the blue track of Usain Bolt in the 100m heats, will tempt more fans to the stadium after the opening day of athletics was watched by paltry crowds.

There were probably fewer than 10,000 fans scattered around the 60,000-capacity stadium, despite organizers claiming to have sold 68 percent of the tickets.

IAAF President Sebastian Coe had said earlier this week that there had been some "scheduling issues" and an evening session with only one final, the women's shot, would have been a hard sell even in a country with a strong athletics tradition.

It was one to remember, however, thanks to Carter, whose father Mike won silver in the shot put at the 1984 Olympics and who became only the second American woman after Earlene Brown in 1960 to medal in the event since it was introduced in 1948.

"He's my coach today, and he's given me everything that he knows to be the best shot putter I can be," Carter said of her father.

EXTRAORDINARY START

The day began in extraordinary fashion when Ayana smashed a 23-year-old world record to win the greatest-ever women's 10,000 meters and was then immediately forced to defend herself in the face of doping allegations.

Ayana clocked 29 minutes 17.45 seconds, an incredible 14 seconds inside the 29:31.78 set by China's Wang Junxia in 1993.

No woman had gone under 30 minutes in the last seven years, but the first four all did on Friday, and the first 13 finishers ran the best time of their lives, including several national records.

Ayana had barely completed her victory lap before the questions started, and in her news conference she denied using performance-enhancing drugs.

"My doping is my training, my doping is Jesus. I am crystal clear," she said through an interpreter.

The day's other medal event was the men's 20km walk, where Wang Zhen and Cai Zelin pulled off a Chinese one-two.

Briton Jessica Ennis-Hill made an excellent start to the defense of her heptathlon title to lead overnight after four events.

Two 21-year-olds - Belgium's Nafissatou Thiam and Akela Jones of Barbados - are leading the chase, with another Briton and strong medal hope Katarina Johnson-Thompson up against it after a shot-put disaster.

Along with the 100 meters action on Saturday, the other highlight should be Briton Mo Farah's bid to secure part one of his attempted double-distance defense when he goes in the 10,000 meters.

(Editing by Peter Rutherford)