By Pritha Sarkar

LONDON (Reuters) - The world's best gymnasts will be hoping the sweat and tears they have shed in training will provide them with the spark they need at the Olympics to pull the plug on two record-chasing champions who have been dubbed "robots".

Such has been the dominance of Japan's Kohei Uchimura and American Simone Biles over the four-year Olympic cycle that if they fail to win the all-around titles at the Rio Games, it will be considered one of the biggest shocks witnessed in the sport.

After collecting every Olympic and world all-around title since taking silver at the 2008 Beijing Games, King Kohei will be favorite to become the first man since compatriot Sawao Kato in 1972 to win successive Olympic all-around titles.

But the all-conquering gymnast, who has been immortalized in comic strips in his homeland and has an army of screaming teenybopper fans around the world, has his eyes firmly set on a medal he has yet to get his hands on -- the Olympic men's team gold.

The collective strength of China means that since 1994, Chinese men have won 10 of 12 world championship team titles and three of the five Olympic golds on offer.

Japan, who captured five straight Olympic golds from 1960 to 1976, did climb to the top of the podium in Athens in 2004 but that was during the pre-Uchimura era.

Since Uchimura's arrival on the world stage, Japan have had to make do with silver medals behind champions China in 2008 and 2012.

But having finally toppled China from their lofty perch at last year's world championships in Glasgow, Uchimura hopes he can now lead his band of Japanese brothers to Rio glory.

"All I can think about is how much I want team gold. That's everything," Uchimura, who owns a record six successive world all around golds, told Reuters.

Despite his diminutive 1.62-metre frame, Uchimura is regarded as a gymnastics goliath as he has turned countless rivals into emotional wrecks in his never-ending search for perfection.

"A lot of foreign athletes say I perform like a machine. I take that as a compliment," said Uchimura, whose Olympic medal haul totals one gold and four silvers.

"To me, moves and performances that are mechanical are perfect. A robot can be more accurate and exact in movement than a human can every time."

While other gymnasts have won more medals and more golds, no one has dominated the sport or maintained their levels of excellence across six apparatus for as long as Uchimura.

In a grueling sport that seems to leave champions on the scrapheap faster than it takes Usain Bolt to run the 100 meters, Uchimura's gymnastics lifespan defies logic.

The 27-year-old is not the only one expected to break records in Rio.

American Biles, who was 11 when Uchimura picked up his first Olympic medal in 2008, has also been an unstoppable force after becoming the first woman to win a "three-peat" of all-around golds at the worlds.

"Everyone says that I’m an alien or a robot but I think he (Uchimura) is," the 19-year-old, who is the most successful female athlete at the worlds with a record haul of 10 gold medals, told Reuters.

"I think what Uchimura has done is very amazing and I don’t think anyone could do what he does."

Yet she is now tipped to pull off a feat no woman has achieved for 20 years.

Since the event was introduced to the Olympic program in 1952, only three women have captured back-to-back world and Olympic all around titles - Larisa Latynina (1960), Ludmilla Tourischeva (1972) and Lilia Podkopayeva (1996).

So why does the 1.45-metre dynamo think no one has done the double since 1996?

"I wasn’t born earlier, so I had no say in that!" quipped the 1997-born gymnast.

Biles and her American team mates -- who include Olympic all-around champion Gabby Douglas -- will be the overwhelming favorites to win a second successive gold having annihilated all before them since 2011.

(Reporting by Pritha Sarkar, editing by Toby Davis)