The semi-finals of the men's 200m butterfly took place in the early hours of the morning.
Michael Phelps qualified for the final by finishing second in his semi-final. Just behind the American in that race was South African Chad Le Clos - the man who beat Phelps to gold in the event four years ago.
You may remember that race for the incredible reaction of Phelps' mother who thought he had won gold when in fact he had been beaten by hundreths of a second.
As Phelps and Le Clos waited for their semi-final last night, the South African appeared to be attempting to psyche out his opponent by shadow boxing right in front of him.
The look on Phelps' face said it all.
On NBC's coverage, they said that they'd never seen anything like it. Phelps was described as 'growling like a dog'.
Post-race, Phelps, while speaking to NBC, denied that he had even been looking at his rival.
I was trying not to really even look at him. He does his thing and I do my thing.
You can watch Phelps and Le Clos in the ready room below.