Andy Schleck closed to within 15 seconds of the yellow jersey with an audacious breakaway victory on the Col du Galibier in the Tour de France.
Schleck was only denied the overall lead by a late surge up the climb from yellow jersey wearer Thomas Voeckler.
Leopard-Trek rider Schleck, runner-up in 2010, raced clear with 60km left on stage 18 to win from brother Frank.
Mark Cavendish was outside the time limit but escaped with a 20-point penalty to retain the green jersey.
An enthralling day's racing on the 100th anniversary of the Tour's first trip up the Col du Galibier has set up an intriguing climax as the riders head to Paris.
Schleck's brother Frank raced clear of the chasers in the closing stages to take second and is now 1:08 adrift in the overall standings, while noted time trialler Cadel Evans is 1:12 behind.
Friday's stage from Modane Valfrejus to Alpe-d'Huez is the final day in the mountains, and with such small margins currently separating the leading contenders, Saturday's individual time trial around Grenoble could prove decisive.
Isle of Man sprint star Cavendish was amongst an 88-strong group who trailed in more than 35 minutes behind the stage winner, outside the margin normally allowed by race organisers.
However, the backmarkers were spared elimination as officials made allowance for the unusally high percentage of riders who strayed beyond the time limit on what was a brutal stage.
Cavendish's grip on the green jersey has been loosened with his lead over Jose Joaquin Rojas in the sprint standings now down to 15 points.
The race organisers set the cut-off point at 33:07 behind Schleck, a time Cavendish's group missed by only 2:33.
"It's disappointing. We thought we were way out of the time limit with quite a long way to go but it was a lot closer and had we known it might have made a difference," said Cavendish.
Andy Schleck's hopes of securing the yellow jersey awarded to the race's overall leader last year were dented when eventual winner Alberto Contador exploited his jammed chain on the 15th stage.
But Schleck took revenge in emphatic style as he surged through the Tour's highest-ever finish 3:49 ahead of the demoralised Spaniard.
Four minutes 44 seconds, an improbable margin to recover over the three remaining stages, now separate Contador from a successful defence.
"Victory is impossible now. I had a bad day," said the Spaniard.
"My legs did not respond and I suffered an incredible collapse. It was a very hard day from the start."
Schleck attacked on the way up the Col d'Izoard, with more than 60km left to the finish, and quickly put two minutes into the peloton, with no immediate response from Contador, Evans or Voeckler.
With older brother Frank marking the other main contenders, Schleck received help from Leopard-Trek team-mates Joost Posthuma and Maxime Monfort.
The huge stakes involved in his gamble became clear as he established himself as the yellow jersey 'on the road', but facing a draining 20km climb to the safety of the finish.
At one point he increased his lead beyond four minutes but Evans led a fightback on the climb up the Galibier.
As Schelck tired the chasing group cut the gap enough to enable Voeckler, who was happy to sit in Evans's wheel for most of the climb, to retain the yellow jersey once again.
The expected strike from Contador, who had been dutifully marked by Frank Schleck up the mountain, never came as he slipped off the back of the chasing group in the last two kilometres.
STAGE 18 RESULT
1. Andy Schleck (Lux/ Leopard Trek) 6hrs 07mins 56secs"
2. Frank Schleck (Lux /Leopard Trek) +2:07"
3. Cadel Evans (Aus /BMC) +2:15"
4. Ivan Basso (Ita/Liquigas) +2:18"
5. Thomas Voeckler (Fra/Europcar) +2:21"
6. Pierre Rolland (Fra/Europcar) +2:27"
7. Damiano Cunego (Ita/Lampre) +2:33"
8. Rein Taaramae (Est/Cofidis) +3:22"
9. Tom Danielson (U.S. / Garmin) +3:25"
10. Ryder Hesjedal (Can/Garmin) +3:31"
1. Thomas Voeckler (Fra/Europcar) 79hrs 34mins 06secs"
2. Andy Schleck (Lux/Leopard Trek) +15"
3. Frank Schleck (Lux/ Leopard Trek) +1:08"
4. Cadel Evans (Aus/BMC Racing) +1:12"
5. Damiano Cunego (Ita/Lampre) +3:46"
6. Ivan Basso (Ita/Liquigas) at same time
7. Alberto Contador (Spa/Saxo Bank) +4:44"
8. Samuel Sanchez (Spa/Euskaltel) +5:20"
9. Tom Danielson (U.S./Garmin) +7:08"
10. Jean Christophe Peraud (Fra/AG2R) +9:27"