OMAHA, Nebraska (AFP) - Michael Phelps won't swim the 200 metres freestyle at the US Olympic trials, leaving the 18-time gold medallist with four events in his push to qualify for the Rio Games.

"My first event will be the 2 (200) fly," he said late on Sunday, as he turned out at CenturyLink Centre to see training partner and friend Chase Kalisz book an Olympic berth with a victory in the 400m individual medley.

After the 200m butterfly, which begins with heats on Tuesday morning, Phelps is also entered in the 100m fly, the 100m freestyle and 200m IM.

Phelps, who turns 31 on Thursday, won the 200m free at the 2008 Beijing Games as part of his historic eight-gold haul.

But he hasn't excelled in the event since coming out of the brief retirement that followed the London Games.

While a strong 200m free swim at the trials might boost Phelps's claim to a 4x200m free relay berth, it's not essential.

Any swimmer on the Rio team can be selected for any relay, and Phelps' coach Bob Bowman, who will serve as head coach of the US men in Rio, said before the trials that a strong showing in other events would be enough to put Phelps in the relay mix.

"I think you have to weigh how he does in these trials," Bowman said. "If he's at his top level, you have to assume he's at his top level across the board.

"We'll see how things go and look at the field and look at what we've got."

Lochte, the 2012 Olympic champion in the 400m IM, was denied a Rio berth as Kalisz led a changing of the guard in the punishing event.

Lochte, hindered by a groin injury suffered on the breaststroke leg in the morning heats, took the final out fast, hoping to build an insurmountable lead.

But 22-year-old Kalisz powered past the fast-fading Lochte on the breaststroke leg and held on over the finishing freestyle to win in 4min 09.54sec.

"I had to go out faster than usual because I couldn't use my legs in breaststroke," Lochte said. "I did everything I could in that race. It just wasn't enough. (I've) just got to forget about it and move forward.

"I'm going to keep working on it day-in and day-out, and hopefully it gets better," added the 11-time Olympic medallist who could still have a chance to qualify for a fourth Olympics in one of four more events in which he's entered.

The 31-year-old said he considered pulling out of the final.

"But it's the Olympic trials," he said. "If I had a broken leg I'd still go out there and swim. I went out there and did my best - it wasn't enough."

Kalisz's time made him second-fastest in the world this year behind Japan's Kosuke Hagino.

Jay Litherland, who swims for the US despite having New Zealand and Japanese citizenship as well, produced the fastest freestyle split in the field to grab second in 4:11.02 and put himself on the road to a first Olympic appearance.

Lochte was third and out of the Rio running a full second behind in 4:12.02.

Kalisz said he could barely remember the race that booked his first Olympic berth.

"Honestly the whole thing went by so fast. I feel like I'm in a different reality right now. I can't begin to explain what I'm feeling right now."

Phelps, whose 400m IM world record from 2008 still stands, has dropped the event from his programme as he pursues a fifth and final Olympic appearance.

"I know how Ryan feels," he said. "That race is tough. That's one of the hardest races you can put your body through."

But Phelps thought Lochte could bounce back, despite the injury.

Maya DiRado won the women's 400m IM in an impressive 4:33.73, with 2012 Olympic silver medallist Elizabeth Beisel more than three seconds behind in 4:36.81.

DiRado's time put her in the top five in the world this year.

In the only other final on the opening night of the meeting, Connor Jaeger won the men's 400m freestyle in 3:43.79, with Conor Dwyer second in 3:44.66.