Team Sky's Edvald Boasson Hagen secured his second stage win of the Tour de France with victory in Pinerolo.
The Norwegian, who also won the sixth stage of the race in Lisieux, proved the strongest of a breakaway group of 14 riders over the stage's final climb.
Frenchman Thomas Voeckler ended an eighth day in yellow, but conceded 27 seconds to his main rivals.
Australian Cadel Evans is his nearest challenger - one minute 18 seconds behind with four stages to go.
Manx sprinter Mark Cavendish put in another solid shift, picking up a point at the intermediate checkpoint and retaining his green jersey.
Team Sky's strong showing in the absence of team leader Bradley Wiggins, who suffered a broken collarbone on stage seven, has also made it a successful Tour for the only British team in the race.
Rigoberto Uran and Geraint Thomas have both held the jersey for the race's best young rider, while Boasson Hagen is emerging as a consistent threat at the end of each stage.
The 24-year-old played his part in a 14-strong breakaway that established an advantage of more than seven minutes over the peloton as they climbed the category-two Col de Montgenevre.
Irishman Nicolas Roche attempted in vain to lead a group of three across the gap as the field climbed over the border and up to the Italian resort of Sestriere.
But, with any gains immediately at risk on a subsequent 48km descent, the general classification contenders were content to save their aggression for Pramartino - the stage's final ascent.
After breaking free at the head of the field, Spaniard Ruben Perez Moreno reached Sestriere with a clear lead and raced solo downhill in a bold attempt to win from the front.
With 20km to go, the lone leader's lead of over a minute was still intact but he was absorbed back into the chasing pack as they climbed for the final time.
French champion Sylvain Chavanel initially took on the pace but he could not shake Boasson Hagen who powered away before nervelessly navigating the winding downhill roads to Pinerolo for his victory.
Back in the field, Alberto Contador tested Andy Schleck and Voeckler with a burst of acceleration.
But the descent proved more profitable for the Spaniard as Voeckler lost crucial time running wide on the corner as he struggled to keep with the downhill pace.
Contador, a three-time winner, and his compatriot Samuel Sanchez looked as if they may make inroads into the rest of their rivals as they pressed in the final kilometres.
But Andy Schleck, who had looked nervous in wet weather on the way down to La Rochette on Tuesday, brother Frank and Evans stuck doggedly to their tail to record the same time.
Contador confirmed that, after getting caught up in a pile-up on the first stage and suffering a knee injury earlier in the race, he had to take some risks in a bid to win a fourth Tour de France.
"It was a very dangerous descent," he said.
"I was off the front, I saw I could attack and the important thing is to pull back time every day."
And Voeckler revealed that Contador's approach was at least a partial success, convincing him to ride unnecessarily aggressively in the closing stages.
"It's a pity, because I saw that Contador, Evans and the Schlecks finished together," reflected Voeckler.
"If I'd taken fewer risks, I would have finished with them."
STAGE 17 RESULT
1 Edvald Boasson Hagen (Nor/Team Sky) 4hrs 18mins 00secs
2 Bauke Mollema (Ned/Rabobank) at 0:40
3 Sandy Casar (Fra/FDJ) at 0:50
4 Julien El Fares (Fra/Cofidis) same time
5 Sylvain Chavanel (Fra/Quickstep)
6 Dmitriy Fofonov (Kaz/Astana) at 1:10
7 Maciej Paterski (Pol/Liquigas)
8 Dmitriy Muravyev (Kaz/RadioShack)
9 Jonathan Hivert (Fra/Saur-Sojasun) at 1:15
10 Borut Bozic (Slo/Vacansoleil) at 2:20
1. Thomas Voeckler (France/Europcar) 73 hrs 24 mins 34 secs
2. Cadel Evans (Australia / BMC Racing) +1.18
3. Frank Schleck (Luxembourg / Leopard) +1:22
4. Andy Schleck (Luxembourg / Leopard) +2:36
5. Samuel Sanchez (Spain / Euskaltel) +2:59
6. Alberto Contador (Spain / Saxo Bank) +3:15
7. Damiano Cunego (Italy / Lampre) +3:34
8. Ivan Basso (Italy / Liquigas) +3:49
9. Tom Danielson (U.S. / Garmin) +6:04
10. Rigoberto Uran (Colombia / Team Sky) +7:36