By Alex Trickett BBC Sport

Tough terrain may lie ahead for Armstrong Stage 11 preview

It is far too early to write a Tour de France obituary for Lance Armstrong.

The mercurial Texan is still sitting pretty at the top of the 2003 general classification.

And he mustered all of his strength to answer the challenges of his usual rivals - Joseba Beloki, Tyler Hamilton and Jan Ullrich - during some tough times in the French Alps.

But with the peaks of the Pyrenees looming, and following faint signs that Armstrong's grip is finally loosening, the time is right to consider his heirs apparent.

Millar: Armstrong can be toppled

Whether it happens this weekend, next year or in 2010, there will come a time when the American is unable to respond to a decisive break.

And these four men could be best placed to benefit. . .

Will Vino be celebrating in Paris? Vinokourov takes dramatic stage

With Beloki out of the Tour, Hamilton handicapped by a broken collar bone and Ullrich looking far from comfortable in the mountains, many are tipping the Kazakh as a possible winner.

"He's all class. He can climb, he's not too bad in the time trial and I think he brings a whole new dimension to the race," said fellow cyclist Brad McGee.

Vinokourov, 29, is certainly having a spectacular season.

After winning the Paris-Nice classic, the Amstel Gold race and the Tour of Switzerland, his Tour de France stage win on Monday carried him to within 21 seconds of Armstrong in the general classification.

But it was the manner of his overall performance in the French Alps that impressed most.

Whenever there was a break, he was there to cover it.

And when Armstrong laboured on Alpe d'Huez, he was one of only two riders to take advantage.

Is Mayo the best pure climber? Mayo outclimbs Armstrong

The other was Spaniard Mayo, who flew up the "hors categorie" climb to win the prestigious stage.

The 25-year-old may need to brush up on his time trial ability if he is to improve on his third spot overall.

But he may just be the best climber to grace Le Tour since Marco Pantani and that gives him the opportunity to take big chunks of time off his rivals in the mountains.

And Mayo is not unproven.

He ran Armstrong close in this year's Dauphine Libere race and is well supported by an Euskaltel team, which boasts two other men - Haimar Zubeldia and Roberto Laiseka - towards the top of the Tour standings

Spanish fans will really get behind their boys in the Pyrenees.

Armstrong (right) keeps his eye on Basso

Italy's finest was one of the finds of last year's race, impressing throughout before walking away with the white jersey as best young rider.

Despite a disappointing season, he has upped his level for the Tour de France and is only two minutes off Armstrong's pace.

If Basso starts to hold his own in the time trials - he finished well down the field in the prologue - he will be a real handful.

And the 25-year-old may one day pip Armstrong in Paris.

Rogers (left) helped Virenque scale Alpe d'Huez

Not a real threat in 2003, the young Australian may have the best shot of all at eventual Tour success.

At the age of 23, Rogers is still serving his apprenticeship.

But he did a fantastic job nursing yellow jersey wearer Richard Virenque up Alpe d'Huez on Sunday.

And he showed blistering time trial speed to place 19th in the prologue - only 13 seconds off Brad McGee's pace.