IRVINE >> Katie Ledecky entered the 400-meter individual medley final at the Phillips 66 U.S. swimming nationals seeking “fun” competition in the grueling event. Kevin Vargas also noticed his opposition in the men’s final, including a European stalwart one lane away.

The most decorated female swimmer on the planet and the up-and-comer from Southern California — who now train together in Florida — each found the racing they desired Thursday at the Woollett Aquatics Center.

Ledecky, 25, closed out her competition schedule this week with her third U.S. title in as many nights by winning a rare game of chase against rival and fellow Olympian Leah Smith.

Vargas, a 21-year-old representing the La Mirada Armada club, punctuated an impressive five-second drop off his personal-best in the 400 IM by capturing his first national title. It was an emotional breakthrough for the former Diamond Bar High swimmer.

“She gave me a big hug afterward,” Vargas said of Ledecky, who recently extended her record for most FINA World Championship medals to 22. “(And) seeing my club coach (Rick Shipherd) really made me kind of teary-eyed because I never got to truly step up at an age-group level, so it was nice to finally show him that it was worth all the time he put into me when I was younger.”

Vargas, a senior-to-be at the University of Florida, outdistanced British Olympian and top-qualifier Max Litchfield by about 1½ seconds to touch first in 4 minutes, 11.45 seconds, good for the 11th-fastest time in the world this year and fourth among U.S. men.

Vargas credited the breakthrough to his work with Florida coach Anthony Nesty on improving his backstroke. “Nesty has really pushed me,” said Vargas, who started Thursday with a lifetime-best of 4:16.54.

Ledecky, the seven-time Olympic gold medalist best known for distance freestyle dominance, rallied from a half-a-body deficit to Smith in the freestyle leg of the 400 IM to win in a lifetime-best 4:35.77, the fourth-fastest mark in the world this year. The time ranks second this year among U.S. women to 16-year-old Katie Grimes, who took second at the recent World Championships with a 4:32.67.

Smith, who has often chased Ledecky in the distance freestyle events, touched second at 4:36.66.

“Leah is one of the few people that I knew I couldn’t be very far behind going into the freestyle,” said Ledecky, who split her final two laps in 1:00.26. “(I had) as much fun as a 400 IM can be. It hurts like something totally different than any other race.”

Rising Shaine Casas of Texas’ Longhorn Aquatics won the men’s 100 butterfly in 50.40, good for third-fastest in the world this year and a tie with Ian Crocker as the third-fastest American in history.

Gretchen Walsh of Nashville Aquatics took the women’s fly in 57.44 to move to ninth in the world this year.