* Briton extends overall lead to almost four minutes * Wins 18th stage ahead of time trial specialist Dumoulin * Five riders still in the mix for second place overall (Adds quotes) By Julien Pretot MEGEVE, France, July 21 (Reuters) - Britain's Chris Froome edged nearer retaining his Tour de France title when he won the 18th stage, a 17-km mountain time trial, to extend his overall lead to almost four minutes on Thursday. The Team Sky rider, aiming to become the first to retain the title since Miguel Indurain in 1995, clocked a best time of 30 minutes 43 seconds on a course featuring the punishing Cote de Domancy, a 2.5-km climb at an average gradient of 9.4 percent. "I really didn't expect to beat Tom today, pacing was key. More...