Britain's Chris Froome, wearing the overall leader's yellow jersey, speeds downhill during the twelfth stage of the Tour de France Photo: Peter Dejong/AP

Bastille Day is a time for fun and festivities but for Chris Froome and his Tour de France rivals it will be a day of danger and suffering on Friday.

For most mortals the prospect of a 101km bike ride through the Pyrenean mountains would seem daunting, but for reigning champion Froome and race leader Fabio Aru, it is a short and punchy stage 13 with pitfalls aplenty.

Froome was off colour on Thursday and lost his yellow jersey to Aru by just six seconds after cracking on the brutally steep final climb to Peyragudes.

He will wear the barely familiar white jersey of his Sky team on Friday as the beaming Italian Aru dons yellow for the first time.

Froome has worn yellow more than 50 times at the Tour but now that Aru has his hands on the jersey, he's determined his depleted Astana team will fight hard to keep it.

"We've already said since the beginning of the Tour that this is a dangerous stage," said Aru.

"Short stages can really hurt because they're done at speed -- with just 100km, there will be many riders in the break and we'll have to follow them.

"I'm expecting a tough day."

Former Vuelta winner Aru leads Froome with Frenchman Romain Bardet, the winner of Thursday's 12th stage, third at 25sec and Colombian Rigoberto Uran fourth at 55sec.

But all eyes will be on Froome to see how he reacts to his unexpected struggles on Thursday.

Bardet for one is expecting the Briton, a three-time Tour winner, to strike back on Friday. 

"We'll have to be careful of Sky, they don't like losing and will try to turn it around," warned Bardet.

He's also not getting carried away by talk of dead kings.

"We remain human beings and if he (Froome) showed his limits, he nonetheless did well to limit the damage and I'm wary about how he'll react."

If the riders will be on edge during the stage, organisers and local police are also nervous at a time of high tension in France.

Last year's Bastille Day celebrations were overshadowed by a jihadist attack in Nice that saw 86 people killed.

Security at the Tour has been ramped up this year, in particular for this stage with specialist anti-terror units following the race in a helicopter and a canine team trained in sniffing out explosives amongst crowds of people also deployed.