MONTREAL—Michael Phelps is getting ready for the U. S. championships in impressive fashion.

Phelps swam a personal-best time of 50.48 seconds in winning the 100-meter butterfly at the Canada Cup, flirting with the world record set here four years ago by American Ian Crocker.

Phelps, competing in the pool used at the 1976 Olympics, improved on his previous best of 50.58 set in the 2008 Olympics in Beijing.

“This pool is still in great shape and it’s fast too, it’s without a doubt fast,” said Phelps, who won one of his record eight gold medals in setting an Olympic record in the event last summer, the only event in which he did not establish a world record.

On Saturday, Phelps narrowly missed Crocker’s world record of 50.40 set during the 2005 world championships.