U.S. figure skating star Nathan Chen put on a dominant performance to clinch his first Olympics gold medal while reigning champion Japan's Yuzuru Hanyu failed to win a medal in Beijing after falling twice on Thursday.

One of the most eagerly anticipated events of the Games came as the IOC refused to comment on a reported doping case involving another figure skater, 15-year-old Russian sensation Kamila Valieva.

Eight golds were up for grabs on the sixth day of competition in the Chinese capital, with the Americans Chen and Chloe Kim, in the snowboard halfpipe, blitzing the opposition to win two of them in style.

An Olympic title had been the only major prize eluding three-time world champion Chen, but he finally won the gold with an overall score of 332.60, while the Japanese duo of Yuma Kagiyama, on 310.05, and Uno Shoma took silver and bronze. Hanyu was fourth.

An elated Chen said: "I'd always of course dreamed about making the Olympics and winning the Olympics but I was like ... I don't know if I can make that happen.

"I haven't really had time to process fully but it's amazing so far."

Hanyu was chasing a third Olympic gold but having started the day eighth after a disappointing short program, the Japanese icon went for broke with a free skate that included an attempt at a quadruple axel – something no skater has ever landed in competition.

That remains the case after the "Ice Prince" tumbled twice, once trying to nail the notoriously difficult jump.

"Honestly, it feels like everything has gone wrong this time around, but I did my best," said the 27-year-old, who only arrived in Beijing on Sunday.

Skating to "Rocket Man" by Elton John, Chen lived up to his nickname "Quad King" in a routine in which the 22-year-old landed five quadruple jumps, to rapturous cheers from the crowd.

Sensational Kim

After the U.S. won its first gold in Beijing with snowboarder Lindsey Jacobellis on Wednesday, it now has two more in quick succession.

The 21-year-old snowboard sensation Kim had the halfpipe title in the bag after only the first of three runs, scoring a whopping 94.00 points to take a virtually unassailable lead.

Kim became the youngest woman to win an Olympic snowboarding medal when she won halfpipe gold as a 17-year-old at the 2018 Pyeongchang Games.

She took a two-year break from competition after her Olympic success as she struggled to cope with the pressures of fame.

"What you guys need to understand is that this isn't an easy sport," she told reporters Thursday.

"It's all about being smart with it, prioritizing my health and just taking it step by step. We'll go from there."

On the ski slopes, Johannes Strolz won alpine combined gold – emulating his father's triumph at the 1988 Calgary Olympics.

Strolz was dropped by the Austrian team not long ago and had to fund himself, even working as a traffic policeman for a time.

"I think I'm a good example of never giving up," said the 29-year-old.

Norway and Germany top the medals table on five golds each, with Austria and Sweden both on four.

Overshadowed

But the action on the snow and ice was overshadowed after Russian media reported that 15-year-old figure skating star Valieva, who led Russia to team gold at the Games this week, tested positive for a banned heart medication before the Olympics.

The positive test could cost Russia the team gold and also threatens Valieva's participation in the individual event that starts Tuesday.

Russian athletes are competing in the Beijing Games as the Russian Olympic Committee (ROC) after the country was banned because of a massive state-sponsored doping scheme at the 2014 Sochi Olympics.

The sample in question was obtained before Valieva won the European championship last month, the RBC newspaper reported late Wednesday.

The drug detected is trimetazidine, a metabolic agent that is prescribed for the treatment of angina and vertigo, according to Kommersant newspaper.

It is banned by the World Anti-Doping Agency (WADA) because it can increase blood flow efficiency and help endurance.

Separately, Iranian alpine skier Hossein Saveh Shemshaki was suspended from the Olympics after testing positive for an anabolic steroid, the International Testing Agency (ITA) said, the first publicly announced doping case at the Games.