Katie Ledecky made it two wins in two nights Thursday, winning the 400m freestyle at the Pro Swim Series in Des Moines, Iowa, in 3min 59.66sec.

Ledecky lowered her season-best time by more than a second as she broke four minutes for the 19th time.

The US freestyle great is one of just three women to break four minutes in the event, along with Australia's Ariarne Titmus and Federica Pellegrini.

Ledecky had opened the meet with the fifth-fastest time ever recorded in the women's 1 500m freestyle - which will be an Olympic event for the first time at the Tokyo Games this year.

On Thursday she left Leah Smith a distant second in 4:06.81.

"I'm really locked in right now," Ledecky said.

With US swimmers gearing up for their Olympic trials in June, Hali Flickinger and Regan Smith put on a show in the women's 200m butterfly.

Flickinger held off Smith to win in 2:06.11, a Pro Swim Series record and fastest in the world this year.

Smith, the 18-year-old who set 100m and 200m backstroke world records at the World Championships last year, was second in 2:06.39.

Zach Apple won the men's 100m free after 2012 Olympic champion Nathan Adrian and reigning world champion Caeleb Dressel failed to qualify for the "A" final.

Apple won in 48.59sec. Adrian won the consolation "B" final in 48.62 ahead of Dressel's 48.68 -- times that would have put them second and third in the "A" final.