After crossing the Pacific Ocean, American Troy Bradley and Russian Leonid Tiukhtyaev met the previous distance record in their helium-filled Two Eagles balloon on Thursday as they flew about 400 miles northwest of San Francisco.
The previous record was set by the Double Eagle V in 1981.
Just hours later, Bradley and Tiukhtyaev reached another milestone when they hit the 8,465-kilometer (5,260-mile) mark, which was the distance they needed to meet to establish a record under international aviation rules.
The distance still has to be confirmed by the Federation Aeronautique Internationale, a process that can take weeks.
"We're not taking any time to celebrate," said Steve Shope, head of mission control. "We have a lot of work we have to do, and we're just taking this flight one hour at a time."
Another record on the way
The Two Eagles pilots began their flight on Sunday shortly before 6:30 a.m. local time (2130 GMT) in Saga, Japan. Bad weather off the US west coast, however, forced the pilots to modify their original route. They are now due to land on sand dunes along the peninsula towards Mexico sometime on Saturday.
The current duration record of 137 hours, 5 minutes and 50 seconds was set in 1978 when Ben Abruzzo, Maxie Anderson and Larry Newman made the first trans-Atlantic balloon flight. The Two Eagles team were expected to surpass what is considered as considered the "holy grail" of ballooning achievements on Friday morning.
The balloon, which is fitted with a massive, helium-filled envelope and a specially-designed carbon fiber-composite capsule, was designed to stay aloft for up to 10 days, but the loss of gas and ballast has shortened that time by a couple of days.
ksb/gsw (AP, dpa)