Every Olympian has to get ready for their event somehow, but for a minute Monday night it looked like Michael Phelps had crossed over to the dark side.

Before his 200-meter butterfly semifinal, Phelps sat stone-faced with his hood up in the aquatics center "ready room." Right in front of him, South Africa's Chad Guy Bertrand le Clos was shadowboxing.

Watch Phelps Qualify for the 200m Butterly Final

Phelps and le Clos have a lot of history between them. Four years ago, le Clos pulled off one of the greatest Olympic upsets when he beat Phelps, who had the lead but glided too long on his finish.

On Monday, Phelps qualified for the finals in second place in his heat, one second ahead of le Clos. But the race will be remembered for Phelps' locked-in, almost furious game face. The hashtag #PhelpsFace was still trending on Twitter into Tuesday morning.