By Steve Keating

OMAHA, Nebraska (Reuters) - Michael Phelps decided to skip the 100 meters freestyle heats at the U.S. Olympic trials but will be back in the spotlight later on Wednesday with a chance to clinch a spot at the Rio Olympics.

The most decorated Olympian of all time with 22 medals, Phelps has now scratched from more events than he has swum so far, having also pulled out of the 200 freestyle on Monday.

Winner of 18 Olympic gold medals, Phelps will now attempt to qualify for the Aug. 5-21 Rio Games in three events -- the 100 and 200 meters butterfly and 200 meters individual medley.

Phelps, who turns 31 on Thursday, will try to give himself an early birthday present by qualifying for a fifth Olympic team when he lines up in the 200 fly final.

After posting the top time in the preliminaries and semis, Phelps is on the brink of a Rio berth and a shot at reclaiming the title he won in Athens and Beijing but surrendered to Chad le Clos at the 2012 London Olympics.

With Phelps focused on the 200 fly, Nathan Adrian took center stage in the preliminaries of the blue riband 100 meters free heats by clocking the best time of 48.43 seconds.

A battling Ryan Lochte, still nursing a pulled groin that hampered his efforts to qualify for a Rio spot in an individual even, scratched from the 100 free semis after putting up a sixth place finish in the preliminaries with 49.13.

Lochte, an 11-time Olympic medalist, injured his groin in his opening race on Sunday and has so far been unable to get to the wall first when it mattered, touching third in the 400 individual medley and fourth in the 200 free.

Conor Dwyer, second in 400m and 200m free, also qualified for the 100 semi-finals along Matt Grevers, who is still dealing with the shock of a third place finish in 100m backstroke that denied him a chance to defend his 2012 Olympic gold medal.

In the women's 200m butterfly preliminaries Cammile Adams took the top time of 2:08.29 but not without a bit of drama when the world championship silver medalist was disqualified for an illegal turn.

But following and automatic review the disqualification was overturned and Adams reinstalled at the top of the results table, her time nearly a full second better than Hali Flickinger who was pushed into second.

"Cam's the type of person who does not do anything wrong, much less a turn," said David Marsh, head coach of the women's team heading to the Rio Games. "She is just this spot on person who works on it (turns) every day."

Kevin Cordes, who has already qualified for the U.S. team with a victory in the 100m breaststroke, moved into position to record the breaststroke double putting up the top time in the 200 preliminaries of 2:08.54.

(Editing by Frank Pingue)