MESA, Ariz. — Michael Phelps is going short, really short, in the second event of his comeback.

The swimmer who owns 18 Olympic gold medals in middle and longer distance races is going to try sprinting. Phelps will compete in the 50-meter freestyle at the Arena Grand Prix on Friday, except he plans to swim his morning heat using a butterfly stroke.

It’s allowed under the rules, and Phelps is doing it to work on his stroke under race conditions, something he hadn’t experienced until beginning his comeback this week after a 20-month retirement.

“If I can get into the rhythm of my normal stroke again in the 50, I’ll be pretty happy,” he said.

Also competing in the 50 free are former Olympic gold medalist Anthony Ervin, Nathan Adrian, and Michael Andrew, a 15-year-old from Lawrence, Kan., who has already broken over 30 national age-group records.

A relaxed Phelps was smiling Thursday after swimming his first race since the 2012 London Olympics, even though he lost in the 100 butterfly to longtime rival and friend Ryan Lochte.

Lochte touched first in 51.93 seconds, second-fastest in the world this year. Phelps was second in 52.13, fourth-quickest in the world. He lowered his time of 52.84 from the morning heats.