By Alan Baldwin

RIO DE JANEIRO (Reuters) - Freestyle queen Katie Ledecky swam the fastest women's 400 meters of the year on Sunday while the U.S. men's team, minus Michael Phelps for now, showed the Australians they mean business in the 4x100 relay.

France's 2012 champion Yannick Agnel failed to qualify for the men's 200 meters semi-finals, however, and finished 19th overall in the heats behind China's pacesetter Sun Yang.

There were boos heard when Russian Yulia Efimova, who won her appeal against a doping ban on the eve of the Rio Olympics, swam the second-fastest time in the women's 100m breaststroke heats. [L4N1AO0FC].

Ledecky, who is aiming for a golden triple in the individual freestyle events, had only herself to beat and led all the way to touch out in three minutes 58.71 seconds.

Closest rival Jazz Carlin of Britain was 4.12 seconds slower.

The American's time was only 0.34 slower than the world record of 3:58.37 she set in August 2014, and it came after a late night helping the women's team to 4x100 silver.

Having swum in both preliminaries and final, Ledecky went to bed 2:45 a.m. and slept until 11 a.m.

"That one felt pretty easy. Hopefully that means I can be faster tonight," said the 19-year-old, with the record clearly in her sights.

"I think my last stroke was a little lackadaisical. I shouldn't have done that, but, whatever. I think it'll be fine tonight.

"I know what my best times are, and I have my goals, and I'm working to get faster and that's what I've been doing all year, and hopefully it all shows up this week," she added.

The U.S. relay swimmers limbered up for what promises to be a knife-edge battle with Australia by beating their old rivals in their heat, but they will have to watch out for Russia whose time in the other grouping was fastest overall.

Phelps, the most medaled Olympian with 18 golds in a collection of 22, could swim in the final with the decision due to be announced an hour before the last race of the day.

The U.S. team rested Nathan Adrian and Caeleb Dressel, first and second in the national trials, with Jimmy Feigen, Ryan Held, Blake Pieroni and Anthony Ervin in the lineup for the preliminaries.

The men's 100m breaststroke and women's 100 meters butterfly are Sunday's other medal races, with Britain's Adam Peaty the man to beat in the former after smashing his world record on Saturday.

Sweden's Sarah Sjostrom is favorite in the butterfly with her own world record looking vulnerable.

Hungarian Katinka Hosszu was also back in action after winning the women's 400 individual medley on Saturday night and qualified as the fourth equal fastest in the women's 100m backstroke heats.

(Additional reporting by Mark Trevelyan; Editing by Alison Williams)