Olympics Rio 2016: Michael Phelps on course for 20th gold as he advances to 200m fly semi-finals

Phelps finished fifth fastest in the heats, behind Hungary's Tamas Kenderesi and Lazslo Cseh, South Africa's Chad le Clos and Australia's Grant Irvine.

However, the Team USA flagbearer has a fight on his hands for another gold after triumphing in the 4x100m relay in the early hours of Monday, with Kenderesi's finish time a whole second faster than Phelps's, at 1:54.73.

The bruises on his right shoulder and upper back were evidence of his fondness for cupping, an ancient Chinese healing practice. He said he's been cupping for some time, before almost every meet, but the trainer went full bore on him and left some of the worst bruises in some time. The right shoulder, he said with a bit of a wince, "is where I hurt the most."

Coming out of retirement in 2014 after a stint in rehab following his second drunk driving arrest, Phelps said the quick turnarounds between the late-night races tailor-made for U.S. television and the early afternoon heats the next day are tough.

He got massages, ice baths, heat applications to flush the lactic acid out of his muscles along with eating a pound of spaghetti, a food he is not keen on and had to force down.

"I guess I got to sleep at 3 a.m. and I was on an 11 a.m. bus. Quick turnarounds," Phelps told reporters.

It's back to his Rio de Janeiro home then for a quick nap, before he returns on Monday night for his butterfly semi-final, where he will be looking to make it to the final on Tuesday and avenge his defeat in the event four years ago to defending Olympic champion Chad le Clos of South Africa.

Meanwhile, compatriot Katie Ledecky also advanced to the semi-finals of the women's 200m freestyle competition, finishing fastest in the heats by almost a second at 1:55.01 to second-place Emma McKeon's 1:55.80.

If Ledecky progresses to the final on Wednesday, as she is widely anticipated to do so, she will be competing for her third medal of the Games having already won gold in the women's 400m freestyle and silver in the women's 4x400m freestyle relay.

In the 200m individual medley, Siobhan O'Connor and Hannah Miley both qualified for the semi-finals for Britain.

O'Connor qualified with the second quickest time, 2:08.44, behind Hungary's Katinka Hosszu.