Mark Cavendish of Britain reacts as he crosses the finish line to win the 19th stage of the Tour de France on Friday. The 110.6-mile stage started in Bourgoin-Jallieu and finished in Aubenas.

After three weeks and thousands of miles of riding, it all comes down to this: a soaring bald mountain in Provence where the Tour de France can be won or lost.

The fabled and dreaded Mont Ventoux today provides a dramatic climax to Lance Armstrong’s comeback Tour — and teammate Alberto Contador is expected to keep the yellow jersey.

The main question in the 103.8-mile Stage 20 from Montelimar to the ascent that Armstrong calls the toughest in France is whether he’ll be on the podium with the Spaniard when the race finishes Sunday on the Champs-Elysees in Paris.

Friday’s relatively flat stage from Bourgoin-Jallieu to Aubenas did little to change the race standings, though an ever-opportunistic Armstrong shaved off four seconds to his deficit to Andy Schleck in second — making up for what he lacks in physical strength with guile.

Britain’s Mark Cavendish won the stage in a sprint, collecting his fifth stage victory at this race — the most by a rider in a single Tour since … Armstrong in 2004.

Contador, like the Armstrong of yesteryear, has been positively dominant at the Tour this year, winning a mountain stage and a time trial — phases of the race that count so much toward overall victory.

Armstrong, returning to cycling’s main event at age 37 and as one of the oldest men in the peloton, has far more than held his own this year: he trails the Spaniard by 5 minutes, 21 seconds. Schleck, of Luxembourg, is 4:11 off the leader’s pace.

While he has an outside — if unlikely — shot at overtaking Schleck, Armstrong’s big concern are those behind him: Bradley Wiggins of Britain is only 15 seconds behind him, and Schleck’s older brother Frank is 38 seconds back of the Texan.

Contador, whom Armstrong and other Astana riders have at times criticized for an apparent lack of teamwork, says his first job is to win the race — but he’ll lend a hand to Armstrong if he can.

With only one big climb left in the race, the 26-year-old Spaniard is all but a lock for a second Tour victory. He also won in 2007.

“My priority is to protect the jersey up to Paris, but if it’s compatible that I help someone from the team — for example, Lance — I’ll do it without question,” Contador said.

For all his prowess over the years, the seven-time Tour champion has never won at Mont Ventoux. If his performances in the Pyrenees and the Alps are any indicator, he’s unlikely to do it this time, either. While solid in the mountains, he has not been among the very best.

Armstrong has a streak on the line: he won at least one stage in each of his run of seven victories from 1999 to 2005 — though he has repeatedly admitted he’s not the same cycling force as he was then.

“Big day 2morrow…” Armstrong wrote on his Twitter account. “The ‘Giant of Provence’ Mt. Ventoux. Know it well, we’re old friends but haven’t always gotten along. Two times 2nd. Ugh.”

The weather could be a factor: The forecast is for winds of 60 to 80 kph on the zigzagging roads to the moonscape-like peak, race organizers said.

Tour planners saddled competitors with Mont Ventoux to maintain race suspense until the next to last race day, featuring four extra-steep patches along a 13.11-mile ascent.

Armstrong is not alone in dreading it.

“L’Alpe d’Huez is a piece of cake against Ventoux,” said Andy Schleck of another fabled climb also known as “hors categorie” — or so tough that it defies classification in cycling’s ranking system.

The final stage Sunday is usually a ceremonial ride on the Champs-Elysees for the rider in the yellow jersey — at times with the champagne flowing even before the finish.