BOURG-SAINT-MAURICE, France — Lance Armstrong mustered one of his strongest showings yet at this Tour de France yesterday, a dazzling burst of acceleration from yesteryear that allowed him to keep second place.

The seven-time champion was so buoyed by the performance that he suggested he could still contend for the yellow jersey if teammate and race leader Alberto Contador has a “bad day.”

Armstrong, speaking after the 16th stage in the Alps, stressed he doesn’t expect that to happen and only a “big shake-up” would allow for such a scenario.

Contador, the 2007 Tour winner, had to fight to retain the overall lead in the 99-mile stage from the Swiss town of Martigny to Bourg-Saint-Maurice, France, which was won by Mikel Astarloza of Spain.

As Contador tried to keep pace with two attackers on the final climb, the 37-year-old Texan lagged. But after dropping back at least 35 seconds, he popped out of his saddle and recovered lost ground.

“I had no choice. . . . So I waited until we had a steeper section and then I got away with an acceleration,” he said.

Contador was impressed, but not surprised.

“It’s easy to explain — he’s a very great rider,” said Contador, who leads his Astana teammate by 1:37. “He was in the past, and he showed it once again.”

Contador and Armstrong finished in a small group of race leaders behind Astarloza.

Armstrong, back at the Tour after 3½ years of retirement, committed himself to help Contador win the three-week race after the Spaniard took the yellow jersey Sunday. Armstrong appeared to shut down his own ambitions then. But at cycling’s main event — which ends Sunday in Paris — anything can happen.

“If there was a massive shake-up and something happened, then I’d have to be strong — to represent the interests of the team,” Armstrong said. “But I don’t think that’s going to happen.”

“If he were to have a bad day, I think I could cover the moves for the team,” he added. “But I don’t think he’s going to have a bad day.”

In the interview, Armstrong was coy about competing next year, saying only: “There’s a pretty good chance I’ll be there.”

But in an e-mail to the AP, Armstrong’s manager Mark Higgins said the Texan will “for sure” be part of the race next year.