"I had some stomach problems these last few days, and last night I had lots of trouble sleeping," Boonen told L'Equipe after finishing a disappointing 56th at 14 seconds behind David Millar in the time trial prologue of the Race to the Sun on Sunday.
"I even asked myself if I could take the start. This morning I had some bad feelings. I did what I could. And 6'15" is not that bad of a time."
Stage one of the ProTour season's inaugural event takes the peloton 186-km from Cloyes-sur-le-Loir to Buzançais with a single category three climb midway through the stage.
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Though the Quick Step sprinter would normally be the favourite in a stage that will almost certainly end in a sprint, Boonen played down his chances of victory on Monday.
"For the sprint, we'll see. People are asking me if I'm going to test myself against [Lampre sprinter Daniele] Benatti," the Belgian said. "I won't test myself."
"Simply, I know that I have a sprint to do. Everyone is waiting for that."
Follow LIVE coverage of stage one from 15:00 CET on www.eurosport.com.
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