AX-3-DOMAINES, France - Chris Froome faced the inevitable questions about doping at the Tour de France after demolishing his rivals to claim the overall lead and said his results would not come under suspicion in the future.

Briton Froome won yesterday's eighth stage, a 195-km mountain ride from Castres, to lead overall by 51 seconds from Team Sky colleague Richie Porte with Spain's Alejandro Valverde in third place 1:25 off the pace.

"There are only several mountaintop finishes; we wanted to go for it, take advantage from a hard finish," Froome told reporters. "I've been in a few leader's jerseys this year but nothing compares to the yellow jersey of the Tour de France."

Double Tour champion Alberto Contador, who could not respond to Froome's attack five kms from the finish, lies in seventh place overall 1:51 back.

His dominant performance was reminiscent of the way the disgraced rider Lance Armstrong would hammer the field in the first mountain stage of the Tour before controlling the rest of the race with often sterling performances in the time trials.

Asked to confirm that his performances had nothing to do with banned substances, Froome said: "One hundred percent. I think it's normal that people ask questions in cycling given the history of the sport. I know the sport has changed. There's absolutely no way I'd be able to get these results if the sport had not changed.

"Results now are definitely a lot more credible. The questions should be asked about people who were winning races maybe five, 10 years ago when we know doping was more prevalent."

While 2012 champion Bradley Wiggins last year lost his cool when grilled about doping and angrily hit out at accusations made on Twitter, Froome remained calm.

"For me it is a bit of a personal mission to show that the sport has changed," he said. "I certainly know that the results I get are not going to be stripped 10, 20 years down the line. That's not going to happen."

Last year, American Armstrong was stripped of his seven Tour titles, won from 1999 to 2005, for doping and then admitted in a television interview in January that he had taken performance-enhancing drugs.

Just as the translator finished saying Froome's words in French, the rider took the microphone again and continued: "Anyone who actually spends a bit of time with the team, with us ... sees that this is months and months of preparation - going to these training camps in altitude all together, the support off the bike from the sport staff, from my fiancee, this is so much preparation that it's not 'wow,' it does add up."