RIO DE JANEIRO — Olympic track and field got off to an astonishing start on Friday when Ethiopia’s Almaz Ayana smashed a 23-year-old world record to win the greatest-ever women’s 10,000 meters and then had to deny suggestions that her performance was fuelled by drugs.

The world 5,000 meter champion, running only her second 10-kilometer race, ripped the field apart to finish in 29 minutes 17.45 seconds, an incredible 14 seconds inside the previously untouchable 29:31.78 set by China’s Wang Junxia in 1993.

Kenya’s world 10,000 champion Vivian Cheruiyot took silver and though she was far behind, her time of 29:32.53 was still the third-fastest ever run. Ethiopian Tirunesh Dibaba, the double defending Olympic champion, took bronze.

No woman had gone under 30 minutes in the last seven years, but the first four all did on Friday, and even though the field was spread all over the track, the first 13 finishers ran the best time of their lives, including five national records.

After a bleak year of doping and corruption controversies it should have been an uplifting start to 10 days of track and field but, such was the magnitude of victory, Ayala was immediately forced to deny using performance-enhancing drugs.

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

Sarah Lahti, who finished 12th in a Swedish record having taken 26 seconds off her own best, questioned Ayala’s performance while marathon world record holder Paula Radcliffe said in commentary for the BBC: “I’m not sure that I can understand that.

“When I saw the world record set in 1993, I couldn’t believe what I was seeing. And Ayana has absolutely blitzed that time.”

Doping remained on the agenda away from the track too as Bulgarian Silvia Danekova’s positive test for EPO was confirmed.

The 33-year-old, who was due to compete in the women’s 3000 steeplechase on Monday, denied taking any drugs and blamed her test on her country’s links with Russia.

“I feel robbed emotionally,” she said. “But we’re coming from the east, we’re too close to Russia.”

Virtually all of Russia’s track and field team have been excluded from the Rio Games following revelations of state-backed doping.

Kenya’s sprint coach John Anzrah was told he could face criminal charges after he was sent home for posing as an athlete in a dope test.

The country’s athletics manager Michael Rotich has already been sent home but he was suspended by the International Association of Athletics Federation ethics commission on Friday following allegations of corrupt practices surrounding anti-doping.

Murray, Nadal both squeak into semifinals

London Olympic champion Andy Murray reached the semi-finals of the men’s tennis on Friday after winning a third-set tie-break to scrape through a testing encounter against American Steve Johnson.

Spain’s Rafa Nadal also secured a spot in the semi-finals with a hard fought victory against local favourite Thomaz Belucci of Brazil, rallying back after losing the first set.

Britain’s Murray, the world number two, edged into the last four with a 6-0, 4-6, 7-6(2) victory but was pushed to the limit by the 22nd-ranked Johnson.

“It was a very tough match and I just managed to turn it around in the end,” Murray told reporters. “For me it has nothing to do with having won the gold in London. For me it’s about trying to win more medals for the country and the team.”

Murray will face Japan’s Kei Nishikori who edged out France’s Gael Monfils 7-6(4) 4-6 7-6(6) in a tense third set tie-break after facing repeated match points.

Nadal, ranked fifth in the world, was tested by No. 54-ranked Belucci in a centre court stadium filled with passionate Brazilian fans, but found his rhythm in the final two sets, to clinch the 2-6, 6-4, 6-2 victory.

Nadal will take on Argentina’s Juan Martin del Potro, who advanced with a 7-5, 7-6(4) win over Spain’s Roberto Bautista Agut, drawing enthusiastic chants from rafters packed with blue-and-white clad fans from neighbouring Argentina.

MacLennan repeats as trampoline champion

Canada’s Rosie MacLennan has made it a habit of rising to the occasion whenever she sees the Olympic rings beneath her while she soars through the air and she became the first trampoline gymnast to win back-to-back golds on Friday.

The 27-year-old had been a surprise champion four years ago and was expected to be upstaged in Rio by Chinese world champion Li Dan or 2008 Beijing gold medallist He Wenna.

During qualifying earlier on Friday, MacLennan trailed Li but she soared to the top of the podium with a gravity-defying routine in the final that earned her a score of 56.465 and wild cheers from the crowd. It was Canada’s second gold in Rio.

Li followed her on to the apparatus and had been expected to leapfrog the Canadian in the standings but when her score of 55.885 flashed up, she realised that not only was she not top, she had also been pipped to the silver medal by Britain’s Bryony Page.

Page, who finished seventh in qualifying, was in floods of tears as she knelt on the floor when she realised that she had earned Britain their first ever medal in trampolining when Li failed to overtake her.

China’s Chen a no-show after doping positive

Chinese Olympic swimmer Chen Xinyi was a no-show at the women’s 50 meter freestyle heats on Friday after Chinese state media said she had failed a dope test at the Rio Games.

The Xinhua news agency, citing the country’s swimming association (CSA), said Chen had tested positive for a banned substance on Aug. 7 in the latest doping scandal to mar competition at the Games.

Chen was listed on the official start sheets on Friday as the heats got under way and then recorded on the results as DNS (did not start).

Controversies over doping overshadowed the build-up to Rio and, far from dying down as events got under way, have flared anew as U.S. and Australian competitors have branded their Russian and Chinese rivals drug cheats.