Sir Bradley Wiggins says he must ride a flawless time trial if he is to beat German rival Tony Martin to world championships gold in northern Spain.
"This year's course is a lot different to last year's, which was long and flat," Wiggins said.
"You have to think more on this course. There's no room for error.
"You need to have enough in the tank for the end. It's challenging, a true test of the time trial."
Wiggins goes for gold at 2.34pm (BST), two minutes ahead of final rider Martin. Essex-born rider Alex Dowsett, tipped for a top-ten finish, sets off at 2.18pm.
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Cyclist Chris Froome has tweeted that it is "time for some R & R" after an MRI scan revealed he had fractured both his left wrist and his right hand.
Last year's Tour de France winner sustained the injury when he fell during this year's race.
MRIs done, confirmed fractures to the left wrist & right hand. Time for some R&R...
He decided to withdraw from this year's Tour after falling on three separate occasions.
The UK's governing body for cycling has said high levels of air pollution in some British cities are further evidence of the need to expand cycling.
It follows a study from the World Health Organisation that found nine British cities and towns had unsafe levels of air pollution.
Martin Key, campaigns manager for British Cycling, said: "With almost daily news stories about worrying levels of air pollution, it is clearer than ever that more cycling is the answer to many of the problems we face in Britain today."
"If local and national government put sustained and targeted investment into improving our roads and making them fit for cycling, we will without doubt create healthier, happier communities and more pleasant places to live," he added.
A British woman on a round-the-world cycle trip has been killed in South America after being hit by a van near Bolivia's border with Chile.
Sharon Bridgman, from Devon, had been cycling around the world since June 2012 with her husband Tim.
Robert Szembek, the chairman of Okehampton Cycling Club, which the couple belonged to, said they had been on what was meant to be a "a great adventure".
"It is not what you expect to hear, especially when they went right through Africa and rode through some doubtful areas known for problems and had taken precautions like not camping by the road," he said.
Mrs Bridgman, who is believed to be 38, was cycling a short distance either ahead or behind her husband and another couple when she was hit by the vehicle, Mr Szembek understands.
A Foreign Office spokeswoman said it was aware of the death and is providing assistance to the family.
Sir Dave Brailsford has left his role as British Cycling performance director, Press Association Sport understands.
Brailsford has led Britain's cyclists to an unprecedented period of success, with eight gold medals at the Beijing and London Olympics and has arguably been more influential than any other person in turning cycling into a mainstream sport in this country.
The 50-year-old Welshman also established Team Sky, winning the Tour de France through Sir Bradley Wiggins in 2012 and Chris Froome in 2013.
Now Brailsford has decided to concentrate on his role as Team Sky principal, leaving British Cycling looking for a new figurehead in the lead up to the Rio de Janeiro Olympics in two years' time.
A man who was caught cycling on the M25 in Surrey, after an app on his phone sent him there by mistake, has been fined.
Surrey Police tweeted that the man had been looking for a shorter route to cycle home from work during rush hour. He was fined £50 by the force.
Cyclist stopped and after a long conversation he has been safely removed with his copy of his ticket for £50.
@chris_caulfield a male was looking for a shorter route to cycle home from work. Phone app sent him on motorway so thought it ok. £50 fine
The NHS could save at least £250 million every year if 10% of the trips in England and Wales were made by bike, researchers at Cambridge University found.
Research commissioned by British Cycling found there would be an almost 5% reduction in the number of sedentary lifestyle related illnesses, like heart disease, diabetes and stroke.
Dr James Woodcock, a senior researcher at Cambridge University's centre for diet and activity research, said: "Cycling is a great way for people to embed physical activity in their everyday lives.
"If we can get people to stay active throughout their lives then it can make a huge difference to their health."
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