Seven-time champion Lance Armstrong is facing a tough test as the first Alpine stage of the Tour de France has gotten under way between Pontarlier and the Swiss ski resort of Verbier.

Fourth-placed Armstrong was 8 seconds behind overall leader Rinaldo Nocentini as riders set out Sunday on the 128.9-mile 15th stage.

Armstrong's rivalry with Astana teammate Alberto Contador of Spain is likely to re-ignite after a week without any major shake-ups in the overall standings.

"Expect fireworks. Lots of them," Armstrong wrote on his Twitter feed ahead of the stage.

Contador is third overall, 6 seconds behind Nocentini.

Belgian star Tom Boonen did not start the stage after having a high fever and vomiting overnight, Tour organizers said. He had not been a factor in this year's race, after being cleared to compete the day before the race started.

Tour organizers had sought to prevent Boonen from riding following his two positive tests for cocaine in less than a year, but French sporting authorities ruled in his favor.