Chris Froome moved a significant step closer to Tour de France glory as he retained his overall lead on arguably the most difficult stage of this year's mountainous route.While Rui Costa raced clear to win his second stage this week in the rain in Le Grand-Bornand, Froome held off late attacks from his rivals Alberto Contador and Nairo Quintana to remain five minutes and 11 seconds clear of Contador.Froome must now face one more mountain challenge, tomorrow's 125km stage to Annecy-Semnoz, before he can plan a victory parade in Paris on Sunday night.But to have come through today's stage - described by several riders as the most difficult of a Tour which has already featured Mont Ventoux and a double ascent of the Alpe d'Huez - and to do so in wet conditions towards the end, will be a huge relief for Froome.He finished more than eight and a half minutes after Costa, who had caught Frenchman Pierre Rolland on the last of the five categorised climbs, and led over the top of the Col de la Croix by a minute in heavy rain.The Movistar rider then held his nerve down the slippery descent to finish 49 seconds clear of Andreas Kloden.Costa was also a winner earlier this week on stage 16 to Gap.The day was notable for the lack of significant moves from Froome's rivals with time running out for them to take chunks out of his advantage going into the final weekend of the Tour.Although Contador and Quintana both tried to follow Joaquim Rodriguez off the front of the peloton on that final climb, a move quickly covered by Froome, they had otherwise looked unable to trouble the yellow jersey on a route which included two hors categorie climbs and two category ones.It meant a day of little drama for Froome, who yesterday had a sugar crash on the second ascent of the Alpe d'Huez, and was then hit with a 20-second time penalty for an illegal feed inside the final six kilometres when he took an energy bar from Richie Porte to address it.