LES DEUX-ALPES, France -- For Lance Armstrong, every second counts in the three-week Tour de France.
Spanish rival Joseba Beloki surprised him briefly in Tuesday's 15th stage - the longest of the race at 140.43 miles - by surging ahead in the final stretch.
Such a late challenge could only have taken a few seconds off Armstrong's big overall lead, but even that was too much for the American.
He gave chase and crossed the line just behind Beloki, clocking the same time. His overall advantage of 4 minutes, 21 seconds over Beloki was unchanged, and he remained on course for a fourth straight title.
"Les Deux-Alpes, it's not too hard, it's not good for attacking," Armstrong said after the stage, which Colombian rider Santiago Botero won in 5:55:16.
Armstrong was ninth, 6:41 off the pace.
The mountain stage from Vaison-la-Romaine in the southern Provence region to this ski station was the first in the Alps this year. But it wasn't as hard as the two earlier legs in the Pyrenees - both of which Armstrong won - or the trek to the top of the Mont Ventoux, in which he extended his lead.
Also, it was much easier than Wednesday's stretch from Les Deux-Alpes to La Plagne, which features three exceptionally difficult climbs.
Armstrong prefers to be aggressive and attack in the harder stages, in which rivals are less likely to match his pace. At the Plateau de Beille in the Pyrenees and on the Ventoux, the Texan left Beloki stranded as he sprinted to the summit.
The ride to La Plagne takes Armstrong over the Col de la Madeleine mountain pass, which he tackled last year in one of his biggest stage wins. Memorably, he grimaced in apparent pain during that climb, then sprinted up the last stretch to L'Alpe d'Huez. He later acknowledged he had bluffed his rivals into thinking he was in difficulty.
Botero's win was his second in this Tour. He stunned Armstrong to win the ninth stage, an individual time trial.
However, Botero is notoriously inconsistent. He finished a whopping 15 minutes off the pace in Sunday's climb up the Ventoux, and is seventh in the overall standings, 11:31 behind Armstrong.
"Today (Tuesday) everything went perfectly, but I'm very disappointed I lost so much time on my 'off' day," Botero said.
Belgian rider Mario Aerts was second on Tuesday, 1:51 behind Botero, and countryman Axel Merckx was third, 2:30 off the pace.
French rider Christophe Moreau dropped out after crashing early in the stage and cutting his lip. The Credit Agricole leader, who placed fourth in the Tour two years ago, had a string of crashes in the first week and was 41st overall when he quit.