While Lance Armstrong rides to what will surely be his fourth Tour de France title, the man who for years has been his chief rival sits on the sidelines and is now facing a doping ban.

Jan Ullrich, runner-up in four of the last five Tours, has been banned for six months and fined after a positive test for amphetamines.

The German rider admitted taking the drugs at a nightclub, and according to the disciplinary panel, he didn't use them to enhance his athletic performance.

Ullrich was the second-place finisher in two of Armstrong's three titles. Ullrich finished seven minutes behind the American last year, but had to skip this year's event because of a knee injury.

Meanwhile, Armstrong, seeking his fourth straight championship, extended his lead to 4:21 minutes after 15 stages of the 2002 Tour.

Colombian rider Santiago Botero won Tuesday's leg.

Botero completed the 226.5-kilometre trek from Vaison-la-Romaine to the Les Deux Alpes in the French Alps in a time of 5:55:16.

Belgian rider Mario Aerts was second, 1:50 behind Botero, and countryman Axel Merckx was third, 2:29 off the pace.

Armstrong finished in a pack that included this campaign's main challenger, Spain's Joseba Beloki, more than six minutes behind.

It's a good bet that Armstrong will extend his lead during Wednesday's 16th stage.

The 179.5 km ride from Les Deux Alpes to La Plagne features three exceptionally difficult climbs -- Armstrong's forte.

"(Stage 15) was a bit long, but we had no problems," Armstrong said.

"Les Deux Alpes is not a very hard climb, it's not a climb on which you can break away. (Wednesday) is much harder, it's the classic climbs of the Tour."

Armstrong assumed the leader's yellow jersey last week by winning the Tour's first mountain stage. He built on that lead as the riders moved through the Pyrenees.

Armstrong, a 30-year-old Texas native, who recovered from testicular cancer to win the Tour de France each year from 1999 to 2001, is looking to become the first American rider to win four Tour titles, surpassing Greg LeMond's three triumphs in the event.

The Tour concludes on July 28 with a ride through Paris ending at the Arc de Triomphe.