Evans is the first Australian to wear yellow since 2004
Leonardo Piepoli of Saunier Duval won the 10th stage of the Tour de France from Pau to Hautacam while Australia's Cadel Evans claimed the yellow jersey.
Silence-Lotto's Evans took the race lead from Kim Kirchen, while Luxembourg's Frank Schleck is only one second behind the Australian.
Veteran Italian Piepoli beat team-mate Juan Jose Cobo of Spain into second.
Schleck came third on a day when many of the pre-race favourites found this year's first major climbs tough going.
The day's biggest loser was Alejandro Valverde, the Spanish national champion seen as a potential title threat going into the race.
The Caisse d'Epargne team leader could not keep up with his main rivals in the first climb up Tourmalet and lost further time up the Hautacam to finish four minutes 41 seconds behind Evans overall.
Like the others who took the yellow jersey on the Hautacam, I hope I can continue in it
Luxembourg and Team Columbia's Kirchen fell to seventh place at 1min 56secs behind Evans, who will be thankful of a rest day on Tuesday having injured his shoulder in a heavy crash on the ninth stage.
The other big losers of the hardest stage in the Pyrenees were Andy Schleck, who is now 8min 34secs off the lead, and Italy's Damiano Cunego, who is more than five minutes behind Evans.
Evans, who finished second overall last year, is the first Australian to pull on the yellow jersey since sprinter Robbie McEwen wore it for a day in 2004.
In the three Tours to have stages that finished at the Hautacam, the rider who emerged from the punishing 14.4km ascent in the yellow jersey kept it all the way to the finish in Paris: Miguel Indurain in 1994, Bjarne Riis in 1996, and Lance Armstrong in 2004.
Evans said: "I thought my Tour was finished yesterday. Like the others who took the yellow jersey on the Hautacam, I hope I can continue in it."
Remy di Gregorio, a 22-year-old Frenchman looking for a popular Bastille Day win, led the pack over the 17.7km Tourmalet pass - the longer of the stage's two massive climbs.
But the group of favourites gobbled up Di Gregorio early in the climb up the Hautacam, with just under 12km to go.
Britain's Mark Cavendish of Team Columbia, who won the fifth and eighth stages, collided with America's Danny Pate early in the stage.
Cavendish was treated by the race doctor for an injury to his left shoulder, finished 169th and last and is now almost two hours behind Evans overall.
Stage 10 results, Pau to Hautacam (156km):
1. Leonardo Piepoli (Ita/Saunier Duval) 4 hours, 19 minutes, 27 seconds.
2. Juan Jose Cobo (Spa/Saunier Duval) same time.
3. Frank Schleck (Lux/Team CSC) 28 seconds behind.
4. Bernhard Kohl (Aut/Gerolsteiner) 1:06.
5. Vladimir Efimkin (Rus/AG2R) 2:05.
6. Riccardo Ricco (Ita/Saunier Duval) 2:17.
7. Carlos Sastre (Spa/Team CSC) same time.
8. Cadel Evans (Aus/Silence-Lotto) same time.
9. Denis Menchov (Rus/Rabobank) same time.
10. Christian Vande Velde (US/Garmin Chipotle)
132. David Millar (GB/Garmin Chipotle) 33:14.
169. Mark Cavendish (GB/Team Columbia) 34:55.
Overall standings (after 10 stages):
1. Cadel Evans (Aus/Silence-Lotto) 42 hours, 29
minutes, 9 seconds.
2. Frank Schleck (Lux/Team CSC) :01.
3. Christian Vande Velde (US) Garmin Chipotle,
4. Bernhard Kohl (Aut/Gerolsteiner) :46.
5. Denis Menchov (Rus/Rabobank) :57.
6. Carlos Sastre (Spa/Team CSC) 1:28.
7. Kim Kirchen (Lux/Team Columbia) 1:56.
8. Juan Jose Cobo (Spa/Saunier Duval) 2:10.
9. Riccardo Ricco (Ita/Saunier Duval) 2:29.
10. Vladimir Efimkin (Rus/AG2R) 2:32.
48. David Millar (GB/Garmin Chipotle) 35:22.
169. Mark Cavendish (GB/Team Columbia) 1.44:41.