Description of the problem in the MSDN: 

https://msdn.microsoft.com/ru-ru/en-en/library/aa363858(v=vs.85).aspx 

"To specify a COM port number greater than 9, use the following syntax: "\\.\COM10". 

This syntax works for all port numbers and hardware that allows COM port numbers to 

be specified."

But there is not quite right :) They write "\\.\COM10" it's not going to work.

Correctly so: "\\\\.\\COM10"

I tested on Win7x32, COM > 9. Work correctly.