BEIJING, China (AFP) — The most decorated female athlete in IAAF history, Allyson Felix, has taken the “really difficult” decision to switch to 400m at the World Championships despite dominating the 200m in recent years.

The American sprinter, 29, has won three world 200m titles and is the reigning Olympic champion, having previously bagged silver at the 2004 and 2008 Games.

But with several Olympic and World Championships 400m-relay gold medals to her name, Felix is ready to test herself over the longer distance.

“I’m excited for the challenge of running the 400m, it will be different,” she said yesterday.

Felix missed out on the Moscow IAAF World Championships in 2013 because of injury after overreaching two years previously in Daegu, where she attempted the 200m-400m double but came away with just a bronze and a silver.

“It was a really difficult decision. I felt like I was in the shape that I could do both, but not with the schedule,” she said.

“It’s disappointing. I’m not the only one who can run the 200 and 400m. I wish we had the opportunity to go after it.”

Coached by Bobby Kersee, husband and coach of Olympic champion and world record-holding heptathlete Jackie Joyner-Kersee and the late Olympic champion and world 100m and 200m record holder Florence Griffith Joyner, Felix said stepping up had then been a natural move.

“I had the pleasure and blessing of running the 200m a few times and so we just thought it was time to step up and the opportunity was here,” Felix said.

“Everybody knows I just love the 200m. To think of going to the championships and not being in it is very difficult.

“I’ve had so much experience with the 200m, I know that race very well. The 400m doesn’t come that natural for me.

“The 400m is a very strategic race and I have to learn how to put together the complete race.

“The most difficult part is being patient. I’m a true sprinter, I like to go fast all the time. When you try to slow me down or give me a strategy, it’s a little difficult to lock into that,” Felix said.

Her decision likely means that Francena McCorory, the fastest woman in the world over 400m in the past two years, will miss out on the individual event in Beijing.

McCorory could only finish fourth in the US trials and would only have been able to race at the worlds had Felix opted out.