By Steve Keating

OMAHA, Nebraska (Reuters) - Michael Phelps and Ryan Lochte cruised into the semi-finals of the 200 meters medley at the U.S. Olympic trials on Thursday to set up a potential mouth-watering showdown between the two rivals.

Back in the pool 14 hours after securing a spot on his fifth U.S. Olympic team with a win in the 200 butterfly, Phelps showed no signs of slowing down as he celebrated his 31st birthday by touching the wall unchallenged in one minute, 58.95 seconds.

"I feel older," Phelps said after his swim. "My muscles aren't the same, they hurt a lot worse this morning but this is the grind time for me over the next couple of days, just try to get as much rest as I can and recover the best I can."

The only swimmer to turn in a faster preliminary time was Lochte, who continued to fight through the pain of a pulled groin to top the result's sheet with 1:58.05 and advance to the 16-man semi-final set for later on Thursday.

While Phelps and Lochte still have work to do to reach Friday's final, the prospect of one more showdown between the longtime rivals could prove to be the highlight of the trials.

Phelps, winner of a record 22 Olympic medals, has made the gold medal in the 200 IM his personal property taking top spot on the podium at the last three Summer Games.

Lochte, winner of 11 Olympic medals and runner-up to Phelps in the 200 IM at the 2012 London Games, is the world champion and record holder in the event and still looking to qualify for his first individual event at the Aug. 5-21 Rio Games.

Hampered by a pulled groin he sustained in his opening event of the trials on Sunday, Lochte has qualified for the U.S. team as a member of the relay pool but the 200 IM may represent his best chance to get into an individual event.

"That was a good morning swim," said Lochte. "But I completely changed my breaststroke, I can't do the normal kick I usually do because it hurts so bad.

"I just have to recover and make sure I am ready for tonight."

Katie Ledecky, who is battling Phelps for top billing at the trials and has already secured Olympic spots in the 200 and 400 free, was also back at the CenturyLink Center looking to add the 100 free to her Rio dance card.

Rated the top women's distance swimmer, the 19-year-old flashed her sprinting credentials on Thursday by clocking the sixth best time of 54.04 in the morning preliminaries with an eye toward muscling her way onto the relay teams.

Abbey Weitzeil, yet another of the fresh faces making an impression at the trials, posted the top time of 53.58 in the heats ahead of 30-year-old Amanda Weir, swimming in her fifth trials, in 53.76 and Dana Vollmer, winner of three gold at the London Olympics.

(Editing by Frank Pingue)