By Gene Cherry

Eugene, Oregon (Reuters) - World champion Allyson Felix stayed on course for a rare Olympic double with a solid qualifying run in the 400 meters semi-finals at the U.S. Olympic track and field trials on Saturday.

Still pained by an April ankle sprain, America's most decorated female sprinter eased into Sunday's final with the second fastest time of the day, 50.31 seconds.

Although definitely not at top shape yet, the 30-year-old hopes to make the U.S. team in both the 200 and 400 meters and go for an Olympic double only two women have ever achieved.

The 200 meters Olympic champion, Felix will need a top three finish in both races at the cutthroat trials to qualify for Rio.

"It's difficult to not feel like me," Felix told reporters. "I'm having to go with what I have and change a little bit."

The right ankle, injured while weight lifting, "is not 100 per cent. But I am fighting."

Felix appeared stronger than in Friday's first round, but attributed it to finally competing. Prior to this weekend, she had run only two 400 meters races and no 200s this year.

The 200 meters, which starts on Friday, is still on her schedule.

Only Francena McCorory had a faster time in the 400 semi-finals, clocking 50.28 seconds.

While young Travyon Bromell overshadowed U.S. mainstays Justin Gatlin and Tyson Gay in the men's 100 meters qualifying, Olympic long jump gold medalist Brittney Reese created some major fireworks.

Trailing world champion Tianna Bartoletta, Reese exploded to the longest jump in 12 years, a lifetime best 7.31 meters.

"I knew yesterday when I jumped seven meters easily, I knew that today would be special," said Reese.

Only Jackie Joyner-Kersee (7.49m) has jumped further among American women.

"She can pass me and go even further," said Joyner-Kersee, the 1988 Olympic gold medalist.

Bartoletta, working in a 100 meters heat between jumps, finished second with a wind-assisted 7.02 meters with Janay DeLoach (6.93m) also making the U.S. team.

BROMELL FASTEST

Bromell, the world indoor 60m champion, breezed through 100m qualifying in 9.94 seconds. Gay clocked a wind-assisted 9.97 seconds and world silver medalist Gatlin 10.03.

None of the sprinters expected Jamaican world record holder Usain Bolt, who is battling hamstring problems, to miss the Rio Games.

"It's a tradition," former world champion Gay said of Bolt bouncing back from previous injuries. "2012, last year ... I know he is going to come back strong."

Jenna Prandini breezed to a wind-assisted 10.81 seconds to pace women's 100m qualifiers.

Favorites English Gardner (10.90) and Tori Bowie (10.91) had the next fastest times.

World record holder Ashton Eaton enjoyed a 82-point lead with 4,560 points after the decathlon's first five events, powered by a run of 10.34 seconds in the 100m and a leap of 7.84 meters in the long jump.

Molly Huddle won the women's 10,000 final (31:41.62) and Whitney Ashley claimed the discus title (62.25 meters).

The top three finishers in each event at the July 1-10 trials qualify for the Rio Games.

(Editing by Andrew Both)