Sinkewitz has been provisionally suspended by his team, T-Mobile spokesman Stefan Wagner said.
TV stations ZDF and ARD, two public channels which have been broadcasting the Tour, said they were dropping their coverage "until further notice."
Sinkewitz has five days to decide whether to request a B sample test.
If that also comes back positive, he faces a possible ban.
"It's not possible. I know nothing about it," Sinkewitz said from a Hamburg clinic.
"I am about to have surgery. I can't deal with it now."
Sinkewitz competed in the Tour, but dropped out after crashing into a spectator Sunday.
T-Mobile company spokesman Christian Frommert would not rule out that the telecommunications giant could decide to stop sponsoring the team.
"We'll sit down after the Tour and calmly analyse the situation,'' he said.
"I wouldn't rule anything out.''