British sailor Alex Thomson believes he is unlikely to reel in his French rival despite halving the lead hours before the finish of the solo round-the-world Vendee Globe race.

The eighth edition of single-handed offshore racing's ultimate test is set to finish on Thursday, 73 days after 29 skippers set sail from Les Sables d'Olonne, western France.

Admitting he is exhausted, the 42-year-old Welshman who now lives in Gosport, Hampshire, has not slept for the past two days and said defeating the Frenchman, Armel Le Cleac'h, is probably beyond his reach.

"I don't think I can catch Armel," he told the Vendee Live show on Wednesday.

"There are no real options for me any more, I think my options have run out. It might be possible to catch a few miles but it's difficult for me at the moment.

"Until I can get my autopilot driving on a wind angle it'll be very tricky in the conditions I have."

Yachting legend Sir Robin Knox-Johnston, who was the first person to sail solo and non-stop around the world, said the "friendly rivalry" between the two countries "adds a bit of spice".

But despite Thomson's reservations over a win, Sir Robin said it is all still to play for and the race will come down to one crucial last manoeuvre as they enter the last 300 miles.

He told the Press Association: "His racing has been exceptional, he has got a very fast boat - he and Armel are two of the top sailors at the moment.

"Both of them are very capable, very determined and very experienced. So it is a great competition."

He said the pair have been "pushed quite a long way north", which organisers said has been caused by a high-pressure weather system - effectively blocking the route back to Les Sables d'Olonne.

"It hasn't finished yet," Sir Robin added.

"Anything could happen, all one could say for certain is there are two people out there who aren't going to get any sleep tonight."

Thomson said issues with the wind instruments on his 60ft boat Hugo Boss have prevented the autopilot working properly.

On top of this, a hydrofoil on his vessel was ripped off by an unidentified submerged object early on in the race.

Sir Robin said the absence of this technology means Thomson "won't get the lift" he needs to sail quicker.

Thomson, who is competing in the race for a fourth time, finished the Vendee Globe in third place in 2012 and is on track to better this result.

Dame Ellen MacArthur is to date the most successful British sailor in the event's history, after finishing second in 2001, while leader Le Cleac'h has been runner-up in the past two editions.

If Thomson does win, he will be the first Briton to claim victory in the race.

Clinching his solo title 48 years ago during the Golden Globe competition, the forerunner to the Vendee Globe, Sir Robin is the last Briton to win a solo non-stop around the world race.

He took on a 24-year-old Thomson to skipper one of his yachts during the 1998 Clipper Round the World Yacht Race - co-founded by Sir Robin more than 20 years ago.

On Thomson potentially taking his spot as the latest Briton to win a race of this nature, Sir Robin said: "It is about time someone else did.

"If it was Alex, I would be delighted - because he used to work for me and we are good friends. I would love to see him do it."

Whether it is the British or French who claim victory in this year's Vendee Globe, both Thomson and Le Cleac'h will beat the previous circumnavigation record of 78 days set by Frenchman Francois Gabert in the 2012-2013 edition of the race.