Former champion Ivan Basso of Liquigas powered into contention for the Tour of Italy's pink jersey after a stunning display of fluid climbing on the 222km 15th stage Sunday.
Overnight leader David Arroyo of Spain managed to retain the race lead despite losing over four minutes to Basso on the 10.1km long Zoncolan climb, whose average gradient was a punishing 11.9 percent.
However the Spaniard, an unlikely winner of the second biggest stage race in the world, is now under threat.
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He admitted: "I don't know about winning the Giro, there were a lot of time losses today."
Italian Basso, who dominated the race in 2006 but was handed a two-year doping ban which only allowed him to return in 2008, had started the day over seven minutes behind Caisse d'Epargne rider Arroyo.
But the Spaniard was one of several big names left trailing once the going got tough on the Zoncolan, a beast of a climb featuring passages with gradients of 20 percent, where an earlier four-man break was easily reeled in.
Shaking off the close attention of world champion Cadel Evans in the final 4km, Basso went off on his own to beat the Australian into second and claim his first Giro stage victory since 2006.
"It's been a marvellous day for me," said Basso. "The most beautiful since my return to the peloton (in 2008)."
Evans finished at 1min 19sec, with Italians Michele Scarponi and Damiano Cunego, Kazakh Alexandre Vinokourov and Spaniard Carlos Sastre all taking places three to six.
Although Arroyo leads another Australian, the surprising Richie Porte of Saxo Bank, by 2:35, Basso is third overall at 3:33 and first among those considered bona fide victory contenders.
Cervelo's Sastre, the 2008 Tour de France champion who moved back into contention several days ago after a mediocre start, is fourth at 4:21 with BMC leader Evans fifth at 4:43.
Basso's Liquigas team had sacrificed their own chances of making it to the finish by imposing a punishing pace on the peloton on the way to the foot of the climb.
And the Italian was quick to pay tribute.
"I have to thank the whole team for all the work they did for more than 100km leading to Monte Zoncolan," he said.
"Each member knew what he had to do at a precise moment, and executed it with 110 percent."
Asked about his attack on Evans, Basso added: "I know this climb really well, and I knew I had to approach it with a fast, but consistent, rhythm - a bit like in a time trial.
"But it was a really difficult climb."
Coincidentally, both riders share the same coach -- Italian Alberto Sassi, who was diagnosed with a brain cancer last month.
Sassi said recently the pair are like "sons" to him, and predicted one or the other would win the Giro.
Basso meanwhile had a good word for the Australian.
"He's a great champion, one of the most tenacious riders I know," added the Italian.
"He's hard to beat and he'll be hard to beat till the end."
Monday is a rest day on the race, which resumes Tuesday with a 12.9km time trial.
