Caster Semenya is back.

The 800m star, who has struggled with injury for the better part of three years, posted her fastest time since 2013 at the world championships in Beijing yesterday morning to advance to tonight's semifinals.

Her 1min 59.59sec effort also counts as a qualifying time for next year's Rio Olympics; she had failed to qualify for the 2013 world championships and 2014 Commonwealth Games.

Semenya had to play catch-up coming off the final bend, moving up from second last to steal third place on the finish line.

It was a fast heat, with the runners who finished fourth to sixth also qualifying as fastest losers.

"Running under two [minutes] is an achievement already, qualifying to come [here] is an achievement so there's not much I can say." But she didn't have to say much.

She suffered a serious injury to her right knee after the London 2012 Olympics, where she had won the silver medal.

"My first lap was about 59, which was okay for me. I just had to hang on, run my own race, try to finish as quick as I can."