Vienna, Accompanied by seven pacemakers, Kenya's Eliud Kipchoge started running Saturday in his quest to bust the mythical two-hour barrier for the marathon -- albeit on a specially prepared course in a huge Vienna park.
Wearing a white jersey, the Olympic and world champion set off from a bridge that leads into the Prater park with the course readied to make it as even as possible.
Throngs of fans lined up around the finish line eager to see if the 34-year-old can cross it at 1hr 59min, for a historical first.
"We want to see if he will make it. I would say it is totally crazy, terrific if he can make it," said Nicole Pahl, 48, who herself runs marathons and travelled to Austria from neighbouring Germany to see the Kenyan's attempt.
Kipchoge holds the men's world record for the distance with a time of 2hr 01min 39sec, which he set in the flat Berlin marathon on September 16, 2018.
He tried in May 2017 to break the two-hour barrier, running on the Monza National Autodrome racing circuit in Italy, failing narrowly in 2hr 00min 25sec.
But this time he says he is mentally stronger and more confident.
"I just have to make that click in people's minds that no human is limited," he told a press conference this week.
He said his attempt in the Austrian capital is about "making history in this world, like the first man to go to the moon".
Because of the way the run is being set up and paced the International Association of Athletics Federations will not validate the time as a world record.
The running surface has been partly retarred and readied with other features such as a banked corner that can save time and avoid injury.
Pacemakers are taking turns to support him throughout the 42.195-kilometre (26.219-miles) race.
That includes a 4.3 kilometre-long straight alley, which he runs up and down several times.
"The course is extremely good. I'm happy with the course," Kipchoge, whose family has accompanied him to Vienna, said earlier this week.
Weather conditions are favourable with very low wind speed and no rain.
Fan zones have also been set up along the course. Organisers are only allowing their own cameras to film the run itself.
"It's history in the making. I'm very proud... It will be a motivation to a lot of young people. People are looking up to him," said Nichasius Koech, 37, a software engineer from Kenya who works in Germany.
The course has been prepared so that it should take Kipchoge just about 4.5 seconds more than on a computer-simulated completely flat and straight path, according to analysis by sports experts at Vienna University.
In total, he will only have to descend 26 metres in altitude and climb 12 metres, the experts said.
The founder of the main sponsors, Ineos, British billionaire Jim Ratcliffe, is taking a personal interest in the challenge and himself competes in Ironman triathlons.
The world marathon record has, for the past 16 years, been contested uniquely between athletes from Kenya and Ethiopia. The two nations are also fierce rivals for distance medals on the track.
Kipchoge's record was almost beaten last month in the Berlin marathon by Ethiopia's Kenenisa Bekele, who ran 2:01.41, just two seconds short of the official world mark.