The Rio Olympics may be a swansong for Michael Phelps but the American swimming great is showing no signs of going gentle into the night.

On Monday, the 31-year-old proved he is in fighting form for his 200 metres butterfly rematch with Chad le Clos, the South African who four years ago denied him a third consecutive gold medal in the event.

World and Olympic record holder Phelps led throughout most of Monday's semi-final before being pipped in the last 25 metres by Tamas Kenderesi of Hungary, who finished 0.16 ahead of the American.

Le Clos finished fourth, more than a second behind Phelps, though he had raced in the 200 freestyle final a short time before and won silver.

Phelps, the most decorated Olympian of all time, retired after 2012 but got back into the game two years later.

There have been question marks over his character after he was photographed with a marijuana pipe in 2009 and pleaded guilty to a drunk driving charge in 2014, his second offense in 10 years.

He got off to a good start in Rio, winning his 19th gold on Sunday night in the 4x100 freestyle. Phelps said he was fighting aches and a tight turnaround between the late-night race and the butterfly qualifying heats on Monday.

To deal with the pain he turned again to cupping, a Chinese healing technique, that has left his upper body full of purple bruises, much commented on social media. He also said he thought he could do well on Monday after a quick nap.

It appeared to have worked out just as he planned.

"Getting a two-hour nap at home felt amazing," said Phelps. "Should be a fun race tomorrow."

Le Clos, who beat Phelps by 0.05 seconds in 2012, is relishing the rematch in Rio.

"I am excited. Been excited for a very long time, very, very excited," Le Clos said.