RIO DE JANEIRO (AFP) - Joseph Schooling pulled off a stunning Olympic upset on Friday by beating US legend Michael Phelps in the 100m butterfly to win Singapore's first ever gold medal.

Asian champion Schooling, 21, completed a wire-to-wire victory to deny Phelps a fourth successive title in the event, winning in a Games record 50.39sec.

Phelps, amazingly, tied with both Chad le Clos and Laszlo Cseh for the silver.

"I don't know if I've been in a tie so a three-way touch is pretty wild," said Phelps, whose Olympic gold medal tally remains at a staggering 22.

"I saw a second next to my name then I looked up again and I looked at Laszlo and Chad and I went we all tied for second, that's kind of cool."

The American, who had won four finals in Rio to stretch his record tally to 22, clocked 51.14, along with fierce rivals le Clos and Cseh.

"It's faster than I went four years ago to win, but Joe's tough," Phelps said. "Obviously he's had a great year last year and had a really great last two years, so hats off to him."

Victory in Friday's fly would have given Phelps a 14th individual Olympic title, but Schooling proved too strong.

Schooling, who took bronze at last year's world championships, punched the water and roared with delight as Phelps swam over to pat him on the back before le Clos ruffled the youngster's hair.

Phelps, competing in his fifth and final Games, will be odds-on to win a fifth gold medal in Saturday's 4x100m medley relay - an event the Americans have never lost at the Olympics - to finish his career with a total of 23 golds.