Team Sky’s Froome defied the rain and took advantage of a late bike change to beat Saxo-Tinkoff’s Alberto Contador by nine seconds on the day to increase his lead over the Spaniard to four minutes and 34 seconds in the general classification.
Contador rose to second place after Dutchman Bauke Mollema (Belkin) finished more than two minutes in arrears to drop out of the top three at the expense of the Spaniard’s team-mate Roman Kreuziger.
Kreuziger, of the Czech Republic, finished fourth on the stage behind Joaquim Rodriguez of Katusha, who completed the podium one second behind Contador.
With a winning time of 51'33, Froome’s latest victory comes after second place in last week's opening time trial at Mont-Saint-Michel and mountain victories at Ax3 Domaines in stage 8 and Mont Ventoux in stage 15. The 28-year-old rider also leads the king of the mountains competition.
"I can’t believe it. Three stage wins, the yellow jersey and the polka dot jersey – this Tour has been incredible for me so far," Froome said at the finish.
“It rained a bit for my second descent and I was prepared to concede time today and not take any risks, so it was a surprise to win."
Froome trailed Contador by 11 seconds at the summit of the second of two climbs – but unlike his rival, the race leader decided to switch to a specialist time trial bike for the final 12km downhill run to the finish.
"It could have made the difference," Froome admitted. "I felt when I rode the course this morning that I needed the bigger gear and so it paid off."
Froome was not alone in his decision to switch bikes ahead of the final descent, with the majority of the race favourites opting for the same tactic.
Spaniards Alejandro Valverde (Movistar) and Rodriguez both swapped bikes before going on to set the provisional best times at the finish.
These efforts were both bettered by Saxo-Tinkoff pair Kreuziger and Contador, which threw an element of uncertainty into proceedings – something heightened by the heavy rainfall which coincided with Froome’s descent.
But the British race favourite held on to turn an 11-second deficit into a nine-second advantage to deny Spain their first stage win on the 100th edition of the race.
Billed as the hardest time trial in Tour history, the 32km course featured two sharp Cat.2 climbs with a maximum gradient of 10 per cent and two highly technical descents along sinuous, narrow and potholed roads.
To highlight the difficult nature of the challenge, experienced Frenchman Jean-Christophe Peraud (Ag2R-La Mondiale) crashed heavily on the second descent in an early-morning training ride, cracking his right collarbone in the process.
Ninth place on GC, Peraud decided to continue the race – but crashed heavily on the same shoulder 2km from the finish to complete his horror day. It was one fall too many, and the 36-year-old was forced to withdraw from the Tour in tears.
The early riders down the start ramp in the pretty lake-side town of Embrun, however, benefited from sun, clement temperatures and dry roads, with Dutch national time trial champion Lieuwe Westra (Vacansoleil-DCM) setting the long-standing best time.
Soon after Spain's Jon Izaguire (Euskaltel) ended Westra's two-hour reign at the top, American youngster Tejay van Garderen (BMC) came home to post the new target time just as the first localised showers fell over the first half of the course.
The effect was devastating, with a succession of riders coming home well in arrears – including Pierre Rolland (Europcar) and Van Garderen’s BMC team-mate Cadel Evans, who finished more than five and seven minutes down respectively.
Speaking in the finish area, 24-year-old Van Garderen admitted that he "might just be the luckiest guy here today".
But the rain cleared and the roads had dried a little by the time the top 20 riders rolled down the ramp, with Valverde crossing the line a full 1’21 quicker than the American.
Rodriguez came through all the checks ahead of his fellow Spaniard, but the real race was going on further back down the road between Froome and Contador.
Contador led at the top of the first climb by two seconds before taking a further 18 seconds off Froome after the first, more difficult, descent. But then Froome came into his own on the second climb – and would have made the split quicker than anyone had he not stopped just before the summit to switch his bike.
As it was, the Sky rider was 11 seconds down on Contador and only in fourth place, with both Rodriguez and Kreuziger quicker through the split.
But his decision to change steeds was vindicated – and Froome secured a hat-trick of wins that could well grow larger still with the prospect of three successive stages in the Alps on the horizon.
Belkin pair Mollema and Laurens ten Dam were the big losers of the day, with Mollema slipping out of the top three, and fellow Dutchman conceding his sixth place to Rodriguez. Following Peraud's withdrawal, young Pole Michal Kwiatkowski (OPQS) moved back into the top 10.
Thursday’s 168km stage 18 from Gap to Alpe d’Huez has been billed as the queen stage and features an unprecedented double ascent of the legendary climb, split by a the testing descent of the Col du Sarenne.
With the riders facing the fearsome 21 hairpin bends of Alpe d'Huez twice in one afternoon, fireworks are expected. For Froome's rivals, it could well be the very last chance to put him under pressure.
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- Alberto Contador
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