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By Cassandra Vinograd

It's being billed as a superstar showdown — but a face-off between friends.

Team U.S.A.'s Michael Phelps and Ryan Lochte, the two biggest hotshots in swimming, will compete Wednesday in the 200m individual medley.

Friendly foes: Michael Phelps (L) and Ryan Lochte.MARTIN BUREAU / AFP - Getty Images

They'll swim in separate heats early in the afternoon and both are expected to make it to the semifinals later in the evening.

It could be last chapter in a friendly rivalry for the ages: Phelps plans to retire (again) after the Rio games.

The teammates have long credited each other for upping the ante in the pool, with Phelps saying after the July trials that "we bring each other to a different level."

In doing so they've shaped Olympic history.

The duo helped lead the U.S. to gold in the 4x200m freestyle relay on Tuesday — giving Phelps his 21st gold medal and making Lochte the second most-decorated Olympic swimmer of all time.

Tuesday night though was just the second time Lochte has gotten in the water in Rio, his fourth Games.

The swimmer, who just turned 32, is competing in fewer events this time around. While he swam three individual events plus two relays in London, Lochte’s program was pared down after the U.S. Trials in June, where he qualified to only swim the relay and the 200m individual medley.

“It’s a lot harder because I’m older,” he said before the Rio games opened. “I can’t put my body through certain practices or sets that I used to be able to do.”

Lochte has suffered injuries but also the burn of a spotlight glare for antics outside of the pool, from partying to an E! reality series.

He's facing off against a reborn Phelps who came out of retirement after two highly-publicized DUI's and a stint in rehab.

But Lochte too has changed: He's heading into Rio after shifting cities and even sprucing up his hairdo. And he has no plans to cut his teammate any slack — or to slow down.

"I'm gonna keep doing it until I stop having fun," he said recently.

Phelps goes into Wednesday's race as the favorite: He's won the 200m IM at the past three Olympics, twice ahead of Lochte in second.