RIO DE JANEIRO - Ethiopian distance runner Almaz Ayana smashed a 23-year-old world record to claim gold in the 10,000m before New Zealand shot putter Valerie Adams suffered an agonising defeat as the Olympic athletics competition opened with a bang on Friday.

Ayana, 24, left her rivals trailing as she finished in 29min 17.45sec, slicing nearly 14 seconds off the previous world best of 29:31.78 set by China's Wang Junxia in 1993 during the era of notorious coach Ma Junren. Vivian Cheruiyot of Kenya took silver in a new national record of 29:32.53 while defending champion Tirunesh Dibaba won bronze in a personal best 29:42.56.

Yet the astounding nature of Ayana's victory immediately raised eyebrows in the athletics world. Marathon world record holder Paula Radcliffe was among the first to question Ayana's performance while Sweden's Sarah Lahti, who finished 12th, more than two minutes behind, was also sceptical.

"I do not really believe that she is 100 percent," Lahti was quoted by Swedish media as saying. "There is doubt." Yet the smiling, softly-spoken Ayana was unruffled when asked to comment on suggestions that her performance might not be all that it seemed.

"I praise the lord, the lord gives me everything," she said through an interpreter. "My doping is my training, my doping is Jesus -- otherwise I'm crystal clear."

Ayana's win comes at a troubled time for Ethiopian athletics. The International Association of Athletics Federations in March placed the country in "critical care" with four other nations over failures in their anti-doping regimes.

There was disappointment for Kiwi shot put star Adams, bidding to become the first woman to win three consecutive Olympic titles in an individual event. Adams was beaten after American Michelle Carter conjured a mighty throw of 20.63m on her sixth and final attempt to take the lead.

Adams had one last chance to regain the lead but could only manager 20.39 to leave Carter celebrating gold -- 32 years after her father and coach won the silver medal in the same event at the 1984 Los Angeles Games. "I was just praying to give it my all," said Carter of her winning fina throw. "I wanted to leave nothing in the tank."

Adams was gracious in defeat, and took satisfaction from even making the Olympics after an injury-plagued spell which has seen undergo five surgeries in three years. "A lot of athletes woud have given up and retired. It's bittersweet. But it's sport, you have to take it on the chin," Adams said.

In other action on the track Friday, British heptathon star Jessica Ennis-Hill moved into a 72-point opening day lead after four events as she aims to retain her 2012 Olympic title.

The 30-year-old amassed 4,057 points, with Belgium's Nafi Thiam in second (3,985) and Barbadian Akela Jones third (3,964). But the Englishwoman described the rest of her day as "a mixed bag really" after managing a best of 1.89m in the high jump, 13.86m in the shot put, 23.49sec in the 200m.

"I am really pleased with the hurdles and the high jump. The high jump's the best I've jumped for a long time," she said. "But I'm really disappointed with my shot put and my 200m was a bit down."

In the day's other medal, China's Wang Zhen won the 20km race walk, with compatriot Cai Zelin taking silver and Australia's Dane Bird-Smith bronze. Day two of the athletics competition on Saturday will see Jamaican star Usain Bolt make his entrance in the opening rounds of the 100m. Saturday will also the title of world's fastest woman decided with the final of the 100m.