Ledecky tunes up with Olympic record in 400m free heats

Katie Ledecky set the tone for a potentially historic Rio Games campaign as she broke the Olympic record and threatened her own world mark in the 400m freestyle heats on Sunday.

The 19-year-old American -- gunning for a rare sweep of the 200m, 400m and 800m freestyle titles -- clocked an Olympic record of 3min 58.71sec, the second-fastest in history behind her own world record of 3:58.37.

And she's just getting warmed up.

"That one felt pretty easy, so hopefully means I will be faster tonight," Ledecky said. "We'll see. That's the easiest it's felt going under four minutes.

Ledecky, who could become the first since Debbie Meyer in 1968 to complete the 200m, 400m and 800m treble, earned silver anchoring the US women's 4x100m free relay on Saturday night.

"Coming off the 100 yesterday had to remind myself this is not a 100 so didn't want to spin my wheels too much," she said. "I backed off and kept it easy up front and I know that that's when I swim my 400 the best -- when I can be really strong on the end."

"It sets me up for a good one tonight."

After an opening night of swimming saw the US shut out of gold, Ledecky is poised to change that on Sunday.

And swimming's biggest star, Michael Phelps, is also tipped to make his first appearance in Rio, leading the US men's challenge of Australia in the men's 4x100m freestyle relay.

In the night's other finals, Adam Peaty will be aiming for 100m breaststroke gold a day after he lowered his own world record in the heats.

Sweden's Sarah Sjostrom looked ready to challenge her own 100m butterfly world record in going for gold against 2012 champion Dana Vollmer.

- Sun seeks to rebound -

China's Sun Yang, denied a 400m free title repeat by Australia's Mack Horton on Saturday, continued his quest for gold as he led the way into the 200m free semi-finals with a time of 1:45.75.

Germany's Paul Biedermann, who set the world record of 1:42.00 in the era of the now-banned supersuits, was second-fastest ahead of South African Chad le Clos.

Reigning world champion James Guy of Britain and American Townley Haas were equal fifth-fastest, while London gold medallist Yannick Agnel of France failed to make the semi-finals.

Le Clos, meanwhile, was delighted -- although he remains firmly focused on his upcoming 200m butterfly duel with Phelps.

"Hopefully I can get a medal in this race. I mean I'm not a natural freestyler -- it's not bad for a fly guy."

Sun had hit back angrily at being branded a drugs cheat by Horton, and tensions over tainted swimmers competing at the Games continued to bubble.

Russia's Yulia Efimova, only cleared to compete at the final moment after a positive meldonium test in March that would have been her second doping strike, heard a smattering of boos from the swimming insiders in the crowd after winning her 100m breaststroke heat in 1:05.79.

That was the second-fastest of the morning behind the 1:05.78 of American Lilly King, and ahead of London gold medallist Ruta Meilutyte of Lithuania.

Australian Emily Seebohm said doping was an intractable problem, but not an insurmountable one for clean athletes.

"It's whoever puts together the best race that wins," she said. "It's not always the drugs that win."

Katinka Hosszu, fresh from crushing the 400m individual medley world record on Saturday, returned to book her spot in the 100m back semi-finals with the equal fourth-fastest time.

France's Camille Lacourt topped men's 100m backstroke qualifying in 52.96sec, with the top four swimmers all touching within a tenth of a second.

China's Xu Jiayu was second-quickest ahead of Australian world champion Mitch Larkin.