By Steve Keating

OMAHA, Nebraska (Reuters) - Katie Ledecky swam the third fastest time ever in the women's 800 meters freestyle as she dominated her heat at the U.S. Olympic trials on Friday.

Ledecky, world record-holder in the 800 free, kept something in reserve but still clocked eight minutes, 10.91 seconds in the event in which she is the reigning Olympic and world champion.

Her world record is 8:06.68. She owns the 10 fastest times in her signature event, and is on course to start a hot favorite in Rio in August.

Such was her dominance that Ledecky touched the wall before some swimmers in her heat had made the turn at the other end of pool, finishing 18 seconds in front of Ashley Twichell.

Leah Smith was second fastest to qualify for Saturday's final but was still more than 10 seconds back of Ledecky in 8:21.64.

Ledecky, who has already qualified for Rio with wins in the 200 and 400 free, has shouldered a massive workload at the trials, entered in every freestyle event from the 50 to 800.

The 19-year-old will be back at the CenturyLink Center later on Friday for the 100 free, where she will be focused more on getting into the relay pool than getting home first.

"This is a little different; I don't think I've done this before, 800 free prelim and then a big time 100 free final, it will be fun, a little different," said Ledecky, looking barely fatigued after her sizzling performance.

"We all want to get faster for that relay so we can be in contention and hopefully I can be part of that this summer."

Ledecky's swim diverted the spotlight from Michael Phelps who eased through his 100 meters butterfly heat.

The Olympic champion and world record holder did no more than he had to do in posting the sixth best time of 51.87, conserving his energy for the evening semi-finals and a much anticipated showdown with old foe Ryan Lochte in the 200 meters individual medley final.

The 200 IM final is expected to be the highlight of the trials as Phelps and Lochte renew one of swimming's all-time great rivalries for perhaps the last time.

Phelps, winner of a record 22 Olympic medals, including 18 gold, has made victory in the 200 IM his personal property, taking top spot on the podium at the last three Games.

Lochte, winner of 11 Olympic medals and runner-up to Phelps in the 200 IM in 2004 and 2012, is the world champion and record holder in the event and looking to qualify for his first individual event at Rio Games.

Caeleb Dressel, runner-up in the 100 free on Thursday, was also back at work and at the top of the timing table posting the fastest effort in the 50 meters free heats of 21.76 just ahead of Anthony Ervin.

Olympic champion Missy Franklin, who has not been in peak form at the trials, saved the best for last posting the top time in the 200 backstroke heats of 2:09.69 ahead of Maya DiRado, winner in the 200 and 400 IM.

(Editing by Andrew Both)