Michael Phelps keeps reminding people this is, what he believes to be, his last Olympics.

"This is my last, hopefully, prelim swim," Phelps said on NBC. "And I just wanted to get through."

Phelps had just come out of the pool after touching the wall in 51.60 in the 100-meter butterfly. He finished second in his heat to 21-year-old Joseph Schooling of Singapore (51.41).

Phelps has gone on record a number of times saying that Rio is his final stop. Many still wonder if things will change down the road, because even at the age of 31, Phelps is still swimming at a peak level.

"This is it, I'm swimming how I want to and this is a great way to finish," Phelps said.

Michael Phelps Olympics
Michael Phelps reiterated this is his last Olympics. USATSI

Thursday is a huge, busy, taxing day for Phelps. He'll swim the 100-meter butterfly semifinal at night, then close it off with the 200-meter individual medley, which is the marquee showdown with Ryan Lochte. A gold is on the line, and Phelps has won that 200 IM at the past three Olympiads.

Meantime, South African Chad le Clos -- you know, the guy famous for dancing and shadow boxing in front of Phelps earlier this week -- also swam in the prelims, but in a different heat than Phelps. He is the reigning world champion in the 100-meter butterfly. But Phelps beat le Clos in the 2012 Olympics in the 100 fly. So more drama to come there.

And one more note for Team USA: Phelps actually swam slower in the prelims than American Tom Shields, who had the third-fastest time overall with a 51.58 touch.