Lance Armstrong will find it hard to sustain his challenge in the latter stages of this year's Tour de France, Alberto Contador's manager said on Sunday.

Armstrong, seven-times Tour champion, took a promising fourth place in the 8.9-kms prologue in Rotterdam on Saturday, the best result by one of the leading Tour contenders.

Contador, the Spanish defending champion, had to settle for sixth place, five seconds behind his American rival.

But Astana chief Yvon Sanquer suggested Armstrong would struggle in the mountain stages, when Contador is expected to thrive.

"We expected Lance to be the man to watch in the first week because it is the time when he can take some time on Alberto," Sanquer told Reuters before the first stage between Rotterdam and Brussels on Sunday.

"It will be much more difficult in the later parts of the Tour."

Armstrong, 38, described his ride on Saturday as his best time-trial performance since coming out of retirement and Sanquer noticed the change.

"I was impressed by Lance's performance in the prologue but not surprised," Sanquer said.

"I was close to him at the start and I saw the determined gaze of old in his eyes. He is still a great champion and still one of the best in time trials.

"We knew that Lance would take chances in the first week and attack us if he could. His prologue performance is a warning. This is not psychological warfare but he had a point to make."

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