Wayde van Niekerk used to pretend he was a famous Brazilian soccer player like Ronaldinho or Ronaldo.

But the 400m world champion will need to be Wayde van Niekerk when he opens his Rio Olympics campaign on the first day of the track and field competition today.

Van Niekerk, a soccer fan, competes in the first round of the 400m tomorrow (2.05am SA time).

"I'm a crazy Brazilian football fan and I think since I was a youngster I thought I was Ronaldinho and Ronaldo.

"But now I've got the opportunity to represent my country right here in Brazil and why not make the most of it before I go out and see Brazil in a lighter way?"

First, however, there's the business of sprinting, lactic acid, pain and hopefully success in what could be one of the most competitive events on the athletics schedule.

His main rivals remain London 2012 champion Kirani James of Grenada and Beijing 2008 king LaShawn Merritt of the US.

The 400m final is on Monday (3am SA time).

In his build-up to the world championships last year, Van Niekerk had clocked a sub-44-second 400m and he'd gone under 20 seconds in the 200m.

This year he hasn't, although he is still unbeaten, having downed Merritt in a one-off 300m race in Jamaica.

"I allowed God to take control and I'm just going to leave it in the Lord's hands and I'm sure that sub-44 will come again.

"I mean, I've done it once before so it shouldn't be a problem to do it again."