INDIANAPOLIS (July 9, 2009)--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, which qualifies him for the world championships later this month in Rome.

The 100 fly is one of the few events that Phelps hasn't dominated.

He'll be chasing Ian Crocker's world record of 50.40 in the evening final.

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.