A medical source also confirmed to AFP the existence of this letter but added that the doctor had appeared "to be in good shape two days ago".
Match-fixing and spot-fixing - determining the outcome of a specific part of a game rather than the overall result - have stained Pakistan sport for several years.
The 34-year-old is just four stage wins shy of equalling Belgian legend Eddie Mercx's record of 34 stage victories in the Tour de France.
The colourful Zhang won silver at the Beijing 2015 World Championships and also competed at the London 2012 and Rio 2016 Olympics.
The 2020 world championship has been unable to start with eight races of a scheduled 22 already either scrapped or postponed.
The double body blow resulted in the entire claycourt and grasscourt seasons being abandoned.