INDIANAPOLIS -- Michael Phelps won the 200-meter freestyle and 200 butterfly at the U.S. nationals Wednesday night, earning himself a trip to the world championships later this month.

Aaron Peirsol created the biggest buzz at the Indiana University Natatorium pool by reclaiming his world record in the 100 backstroke. The Olympic champion trailed Matt Grevers at 50 meters before pouring it on down the stretch to win in 51.94 seconds.

Peirsol became the first person to swim under 52 seconds in the event he has dominated for much of this decade. He has lowered the world record six times.

Peirsol's previous mark of 52.54 set in Beijing was broken by Spain's Aschwin Wildeboer Faber on July 1. Faber swam 52.38 on the backstroke leg of the 400 medley relay at the Mediterranean Games.

Grevers, the Olympic silver medalist, finished second in 53.11, giving himself another crack at taking down Peirsol in Rome.

Phelps held off David Walters in the closing meters of the 200 free and touched in 1 minute, 44.23 seconds. Walters pushed Phelps coming off the final turn, but settled for second in 1:44.95.

Ryan Lochte was third in 1:45.66. The eight-man field included the 800 freestyle relay team of Phelps, Lochte, Ricky Berens and Peter Vanderkaay that won at the Beijing Olympics, one of the record eight gold medals won by Phelps.

Walters, who earned a gold in Beijing by swimming the relay heats, also qualified for Rome. It's the first time the 21-year-old sprinter who trains at Texas will swim an individual event at a major championship.

Phelps returned later to win the 200 fly in 1:52.76, well off his world record of 1:52.03 set in Beijing.

He was pushed in the next lane by Tyler Clary, a 20-year-old Michigan swimmer who stayed close throughout and finished in 1:53.64.

Katie Hoff's second attempt to qualify for Rome ended in defeat again. She finished eighth and last in the 200 free, leaving her with only one more chance in the 100 free. And even that was in doubt, with her coach Bob Bowman saying they would discuss whether she would keep swimming this week.

Dana Vollmer, a 2004 Olympian, won the race in 1:56.20. Allison Schmidt was second in 1:58.63, following up her win in the 400 free Tuesday.

Dagny Knutson, a promising 17-year-old from Minot, N.D., finished fifth.

Hayley McGregory ended years of bitter disappointment with a victory in the 100 backstroke, earning a spot in the first major international meet of her career.

The 23-year-old Texan set a world record in the 100 back prelims at last year's Olympic trials, only to have it taken away by Natalie Coughlin in the very next heat.

In the finals, McGregory finished third behind Coughlin and Margaret Hoelzer, missing out on the Olympic team. She also came in third behind Hoelzer and Elizabeth Beisel in the 200 back at the trials, where only the top two make the Olympic team.

The same thing happened to McGregory at the 2004 trials, where another pair of third-place finishes kept her home from the Athens Olympics.