By Alan Baldwin

RIO DE JANEIRO (Reuters) - Lauren Boyle, New Zealand's top swimming hope at the Rio Olympics, has a problem and it is called Katie Ledecky.

There are racing certainties and then there is the 19-year-old American freestyle phenomenon who looks set for a golden haul in the Rio pool.

The American has smashed the 800 meters world record four times in three years, taking Briton Rebecca Adlington's previous benchmark of eight minutes and 14.10 seconds down to the current 8:06.68.

The fastest time by anyone else this year is 8:18.14 and Boyle's best is 8.25.00 -- some way off her personal best of 8:17.65 when she took silver at the 2015 world championships, more than 10 seconds behind Ledecky.

Reigning Olympic 800m champion Ledecky also holds the world records in 400 and 1,500m freestyle.

Boyle, who has had sickness issues to contend with but is now fully fit and hoping to end New Zealand's 20-year wait for an Olympic swimming medal, preferred to emphasize the positives.

"I think I'm very lucky to race someone like Ledecky and be part of her era and see what she's capable of," the 28-year-old told reporters on Thursday.

"I think it really shows me and other athletes that swim those events what's really possible for women's swimming.

"I see it as a positive, I guess you could see it as a negative if you want to but she's an incredible athlete, she's a lovely girl and I really enjoy racing her," added the New Zealander.

Boyle's own achievements in the pool are considerable. She is already the first Kiwi woman to win three medals at a single world championship (in 2013) and in 2014 she broke the 1,500 freestyle short course world record.

New Zealand has only ever won 10 medals at the world championships and Boyle has provided five of them, but swimming in the Ledecky era means bronze or silver is the best that can be hoped for.

"I guess every time you approach a really big competition it's the same thing, you come up against the best people in the world in your event and that's what sport's about," Boyle said.

"Nobody knows what's going to happen and that's why we do the race. I've come here with an open mind."

The last New Zealander to win an Olympic swimming medal was Danyon Loader, who took golds at the 1996 Atlanta Games in 200 and 400m freestyle.

(Editing by Ed Osmond)