By Nick Mulvenney

RIO DE JANEIRO (Reuters) - Allyson Felix sounded a warning to her 400 meters rivals with a season's best run of 49.67 seconds to reach the final of the one-lap sprint at the Rio Olympics on Sunday.

The American's time was the second fastest of the year behind only the 49.55 clocked in London last month by Shaunae Miller, who was second in the third semi-final behind Felix in 49.91.

Felix, seeking her fifth Olympic gold, got off to a strong start and ran down Miller on the back straight but the Bahamian came back at her on the final bend and might have caught the world champion had she not eased up approaching the line.

"I wanted to get a solid effort and try to make a run at a good lane tomorrow. I wanted to bump up the intensity tonight," the 30-year-old Felix told reporters.

Jamaica's Shericka Jackson ran the second fastest time of the night, a personal best 49.83, to win a tight semi-final ahead of American Natasha Hastings, whose time of 49.90 was also her best of 2016.

The United States and Jamaica also filled the automatic qualifying spots in the opening heat with American Phyllis Francis making up five meters on Stephenie Ann McPherson on the home straight to win in 50.31.

Jamaica's McPherson ran 50.69 for second place. Italian Libania Grenot (50.60) and Ukraine's Olha Zemlyak (50.75) will also line up in the final after qualifying as the fastest finishers outside the top two.

"It was my coach's birthday yesterday," said Zemlyak, whose time was a personal best. "Every day I make him presents. Tomorrow I want to give him the final."

Felix, who suffered an ankle injury in a gym accident earlier this year, was unable to defend her 200 meters title in Rio after failing to finish in the top three at the U.S trials.

The London 400 meters champion Sanya Richards-Ross was also unable to defend the Olympic crown after suffering a career-ending injury at the same meet.

(Editing by John O'Brien)