A solar-powered aircraft flying from Japan to Hawaii on the most perilous leg of a round-the-globe bid has passed the two-thirds point, and was under 40 hours from landing, organizers said.

Solar Impulse 2 had traveled 67 percent of the way to the tropical US state by 2100 GMT Wednesday, having flown 5,199 kilometers (3,230 miles) with 2,536 km more to go, according to the project.

By that time the plane and its veteran Swiss pilot Andre Borschberg had logged nearly 75 hours of continuous flight, having already bettered the previous record of 44 hours they had set between China and Japan.

"Just doing some #yoga exercises on a #solar airplane after 74 hours of flight. That's all. Regular day at the office," organizers tweeted with a picture of Borschberg apparently upside-down in his cockpit.

The Swiss aviator is napping for only 20 minutes at a time to maintain control of the pioneering plane. He is equipped with a parachute and life raft, in case he needs to ditch in the Pacific.

In its latest update, organizers estimated that it had about 40 hours until arriving in Hawaii, if all goes well.

Earlier the plane, on the eighth leg of the global circumnavigation billed as the most difficult part of the adventure, had crossed a potentially problematic cold front, the project said.

The experimental solar-powered aircraft left Japan around 1800 GMT Sunday -- the early hours of Monday local time -- after spending a month in the central city of Nagoya.

The propeller-driven plane was originally scheduled to fly directly from Nanjing in China to Hawaii, but bad weather along the way forced a diversion to Japan that stretched to a month.

Borschberg is alone and entirely self-reliant in the 3.8-cubic-metre (134-cubic-foot) unpressurized cockpit.

Traveling at altitudes of more than 9,000 metres (29,500 feet), he has to use oxygen tanks to breathe and experiences huge swings in temperature throughout the day.

He rests for only 20 minutes at a time and cannot move from his seat, which has a built-in toilet, as well as a parachute and a life raft attached.

Solar Impulse 2 set off from Abu Dhabi earlier this year in a multi-leg attempt to fly around the world without a single drop of fuel.

It has 17,000 solar cells and on-board rechargeable lithium batteries, allowing it to fly through the night.

Its wingspan is longer than that of a jumbo jet but it weighs only 2.3 tonnes -- about the same as a car.