By Steve Keating

OMAHA, Nebraska (Reuters) - Michael Phelps will swim in likely his final race in an American pool while Katie Ledecky is expected to chase a world record as the two biggest attractions at the U.S. Olympic trials take their final bows on Saturday.

The eight days of cut-throat competition to select the U.S. swim team for the Rio Olympics will not end until Sunday with the women's 50 meters freestyle sprint and men's 1,500 free but Phelps and Ledecky will say goodbye looking to add one more event to their Rio Games dance cards.

Phelps, who has already qualified for a fifth Olympics with wins in the 200 medley and 200 butterfly, will close out the trials with the 100 fly, an event he has won at the last three Summer Games.

The U.S. trials have yet to produce a world record but that could change when Ledecky, winner in the 200 and 400 free, takes to the blocks for the 800 meters final where the Olympic and world champion is expected to take a run at her own world record after posting the third fastest time ever in the event in the preliminary heats.

Missy Franklin, the darling of the U.S. team at the London Olympics where she won five medals including four gold, has struggled to find her 2012 form at the trials but will have one last chance to win an individual event in Omaha when she lines up in the final of the 200 meters backstroke.

The bubbly Olympic champion posted the second fastest time in the semi-finals behind Maya DiRado and will need a top two finish on Saturday night to have a shot at defending her gold medal in Rio.

The penultimate day of competition offered up a contrast of events with the women speeding through the 50 meters sprints and the men grinding their way through 1,500 meters marathons.

Madison Kennedy, swimming in her third Olympic trials, closed in on her first Olympic berth clocking the top time in the 50 free of 24.52.

Also qualifying for the evening semi-finals were Kelsi Worrell, winner of the 100 butterfly, Olivia Smoliga, winner of the 200 back and Abbey Weitzeil, the 100 free champion.

Connor Jaeger, who has already qualified in the 400 free, topped the timing table in the 1,500 heats at 14.58.59.

(Editing by Andrew Both)