(CNN) -- Britain's Chris Froome won the 17th stage of the Tour de France on Wednesday to strengthen his grip on the yellow jersey. Froome completed Wednesday's 32-kilometer time trial in 51 minutes 33 seconds, nine seconds ahead of Spain's Alberto Contador with his compatriot Joaquim Rodriguez Oliver finishing a further second behind in third. "I couldn't believe it when I got over the line and saw I had the fastest time," Froome said after his third stage win of this year's tour. "I went into today thinking I was going to try and limit my losses, thinking about the days to come now. So to go through the finish line with the fastest time, I really didn't see that coming." On mountainous terrain along a frequently twisting route most of the riders opted for their traditional road bikes over the more aerodynamic machines used in time trials. But Froome opted to use both on the stage, switching to a time trial model for the final 12-kilometer descent. The swap proved decisive as the Briton clawed back a 20-second advantage Contador had built up earlier in the stage. The Spaniard now moves up to second overall, four minutes 34 seconds behind Froome with just four stages remaining. Contador's Saxo-Tinkoff teammate Roman Kreuziger is now third and trails Froome by four minutes and 51 seconds. Thursday sees the riders tackle the gruelling 172.5-kilometer 18th stage which concludes with a double ascent of Alpe d'Huez over the last 60 kilometers. 
How long did Fromme bike on the 17th stage before the final descent?
20km