Kipchoge starts giving thumbs up to the crowd, inside the last 200m...

500 metres to go and the pacemakers have dropped away. They're cheering on in the background as the Kenyan goes bounding towards the line. What an atmosphere, spine-tingling. He's loving every second of it.

The laser is gone - it's a sprint to the finish!

How does he still have a burst of acceleration in those legs after all that?

Eliud Kipchoge is going to smash the two-hour barrier at this rate...

Former New York City marathon winner Shalane Flanagan: " I truly did not believe I would witness something like this."

Mattpbond: A 4 hour marathon is the dream of normal runners, 3 hours for club runners, 2&1/2 for athletes but 2 hours is just for superheroes.

That's the 40km mark. Splits as precise as a Japanese Shinkansen, and nine seconds to play with.

Eliud Kipchoge is smiling. The Vienna crowd is hollering. This is history in the making...

For any average runner, this is where the lactic acid begins to eat away at your shriveled muscles and the dreaded wall rears into sight, if it hasn't already. You're longing for your mum and cursing why it seemed a good idea to ever sign up to run 26.2 miles in the first place...

Not for Eliud Kipchoge. He's still on for a 1:59:50 marathon, nine seconds under his target, with a three kilometres to go.

Pacemaker Patrick Tiernan: "It's unreal. Five years ago a sub-two hour marathon wasn't deemed possible and now he's cruising around. He's looking great."

2:52 has been the slowest kilometre split so far - that's crazy, and great pacing.

I mean the laser beams help, but what a support team Eliud Kipchoge has.

Here come the final pacemaker team. One last smooth changeover. This is like hitting the final bell... Bernard Lagat will attempt to lead his good friend home.

Still 10 seconds inside the target with six kilometres to go - there is a horde of fans trying to run and cycle along the side of the track, good luck keeping pace with Kipchoge and co...

If you're intrigued by this challenge then I recommend you read Born To Run - it's fascinating.

For those that have, do we reckon a Tarahumara runner could break the two-hour barrier? They'd probably keep going for another 26.2 miles...

Antoine V: It doesn’t matter if it is ‘assisted’ or not. It’s about human achievement. No-one looks back at Hillary and Tergay saying that their climbing of Everest was diminished because they were assisted by their team and using oxygen.

Michael Fisher: I can see this becoming similar to the Cycling Hour Record... a record in it's own but not directly compared to a marathon world record time...

Andy Coombe: Re Rob’s comment (7:53) - my mate holds that record! 159 in 3 minutes since you ask.

Ah yes, that was the world record for eating baked beans with a cocktail stick, right?

Here's Eliud Kipchoge's wife, Grace: "I am nervous but also excited. I am happy for him. Kenyans are very excited for Eliud to do what no human has ever done - they're waiting!"

We're most definitely into the business end of the challenge now. A big 25 minutes coming up for Eliud Kipchoge. Another rotation of pacemakers join the party.

Less than 10km to go...

Image source, Ineos

Normally it's a right palava trying to pick your spot to follow a friend or loved one doing a marathon, but there is no such problem for Kipchoge fans in Vienna - one because there's a load of big screens showing the race and two because he's going up and down the same stretch of road four and a bit times. It's a case of pick your spot, grab some sandwiches and see the Kenyan flyer at least eight times...

Kipchoge is being assisted by 41 pacemakers, who are being helped to maintain a constant speed by a leading pace car that is beaming lasers onto the road.

The pacemakers rotate twice each lap and Kipchoge is being handed his drinks and energy gels from a bike every 3.1 miles, instead of picking them up from a table as in normal marathons.

These aids are not allowed under IAAF rules, which is why athletics' world governing body did not recognise his first attempt as the official marathon world record and will not sanction this one either.

Image source, BBC Sport

That's 30km done. Still on target. Eliud Kipchoge is on course to become the first person to run a sub two-hour marathon...

Sam: I think the 1.59 challenge is brilliant for the sport of athletics, it doesn’t matter if it’s official or not!

Graham Roberts: Profound stuff from Shalane there! Anyone who has run more than two kilometres even slightly against the clock knows about that sort of stuff!!!!

Former New York City Marathon winner Shalane Flanagan: "I got nervous when Eliud looked like he was starting to fall off pace, there looked like there was some strain on his face but he's regrouped.

"It's going to hurt, but I feel like he's setting himself up for some history right now."