INDIANAPOLIS — Michael Phelps was the fastest qualifier in the 100-meter butterfly heats at the U.S. national championships.

He swam the two-lap race in 51.17 seconds Thursday morning at the Indiana University Natatorium. Phelps has already won the 200 fly and 200 freestyle, qualifying him for the world championships in Rome later this month.

Tyler McGill, a 21-year-old Auburn swimmer, was second-quickest and 59-hundredths of a second behind Phelps. Aaron Peirsol, who set a world record in the 100 back Wednesday, advanced in third.

Dara Torres led the 50 free heats in 24.72 seconds. At 42, Torres is nearly double the age of some of the others who made the final.