Michael Phelps registers third triumph at the US Olympic swimming trials ahead of Rio Games

Michael Phelps booked a shot at a fourth straight 100m butterfly gold at the Rio Games on Saturday with his third triumph at the US Olympic swimming trials.

Phelps, fourth at the turn, came off the wall strong and powered home to win in 51.00sec -- making good on his desire to make his last race in a US pool a victory.

"I don`t know if it`s totally sunk in," the 31-year-old said, adding he was "a hair" choked up at the thought he had raced for the last time in front of home fans.

Phelps said he was conscious of that milestone as the race approached, and coach Bob Bowman asked him what his game plan was.

"I said, `I don`t want to lose my last race on American soil,`" Phelps said. "So that was kind of in my head.

"I was determined and fired up today, and this is the best that I`ve felt so far this meet."

Phelps never threatened his world record of 49.82, set in 2009 at the tail end of the bodysuit era.

But he got the job done in booking a third individual event in Rio, where he`ll become the first US man to swim in a fifth Olympic Games.

Phelps, who won the 100m fly at the 2004, 2008 and 2012 Olympics, is now second in the world rankings behind the 50.86 of Hungarian Laszlo Cseh.

"You know, 51.0 is OK, but it`s going to take more than that to win a gold medal," said Phelps, who counts 18 golds among his record tally of 22 Olympic medals.

Although he thought he produced a "terrible finish" with too long a glide into the wall, Phelps found reasons for optimism after coming out ahead of Tom Shields (51.20)."My legs are feeling better," said Phelps, who admitted he was exhausted after a 200m medley win on Friday followed by a third-place finish in his 100m fly semi-final that left him sixth overall going into Saturday`s final and swimming out in lane seven.

"I have a lot of emotion here with Boomer and with the family here," said Phelps, who pointed to fiancee Nicole Johnson and their two-month-old son in the stands after checking the scoreboard.

"There is a lot going on, but I`m happy that we did everything that we wanted to do. I made three events and (have the) possibility for relays, and we`ll see what happens."

Shields, who was seventh at the 50m mark, said he wasn`t surprised to see the "1" by Phelps`s name.

"He is who he is and you`ve got to give him that respect," he said.

Maya DiRado won the women`s 200m backstroke in 2:06.90, booking a third Rio event after victories in the 200m and 400m medleys.

Missy Franklin, who set a world record in winning gold in London, clawed her way into a Rio berth with a runner-up finish.

Franklin, a four-time gold medalist four years ago who crashed to a seventh-placed finish in the 100m back, was fourth at the halfway point but rallied to finish second in 2:07.89 and give herself a chance to defend the 200m crown.

Nathan Adrian, already booked to defend his 100m free title in Rio, won the 50m free by one one-hundredth of a second, clocking 21.51sec to Anthony Ervin`s 21.52.

Ervin, 35, will be challenging for the 50m title in Rio 16 years after winning 50m gold in Sydney in 2000.