Nikolay Davydenko's lawyer insists his client is "truly innocent" of the match-fixing allegations against him and maintains the world number four cannot be forced to hand over his telephone records to the Association of Tennis Professionals (ATP).
Former Scotland Yard detectives working for the governing body of men's tennis flew to Frankfurt to interview Davydenko's wife and brother about the allegations this week.
The investigators are leading the ATP's inquiry into the Russian's retirement through injury from a second-round match against the Argentinian Martin Vassallo Arguello in the Poland Open in August.
Online betting exchange Betfair declared void £3.5million of bets on that match, after identifying suspicious patterns of gambling.
Asked about the phone records on BBC Radio Five Live's Sportsweek programme, Professor Frank Immenga said: "They have to sign the ATP code, the players, and they have to hand out telephone records.
"I have to say that I believe that it is unreasonable to have these kinds of things done by organisations.
"The other question, and where I am getting afraid, is that he has to bring forward the telephone data of all phones he used.
"This means he would have to bring forward my data when he used my phone, if he used the phone of a hotel he would have to bring forward that data.
"I truly believe that these kinds of requests asking for data from third parties is just not possible. They cannot force him into providing data from other people. It is really a legal question.
"Even an organisation like the ATP, responsible for its players, can't go on like that with its players."
Immenga conceded that nine people stood to make more than £700,000 from the match in Poland when Davydenko retired, but insisted phone records would reveal "nothing."
"I know that my client is truly innocent, so they will reveal nothing," he added. "Davydenko was hurt; he had something on his foot; this was already seen before the match. He was sitting in the locker room and having treatment.
"About 50 or 100 people were walking by this room and could check this out. Then on the court itself, he was treated twice - and showing signs of resignation to his wife. It was quite obvious he was not feeling well.
"There are people looking for information, and this is exactly the information they are looking for. When they find out that somebody is already hurt they are going to bet against him. That is exactly what happened, to my understanding."
Davydenko was fined £1,000 by the ATP for not trying hard enough in a match in St Petersburg last month and served 10 double-faults during his third-round defeat in the Paris Masters.
Immenga feels his client is being picked on by the match officials and has twice written to the ATP asking for an explanation.
He added: "For me, it is unbelievable. It is so unfair, what has happened. If you take a look at the code, you will see that the officials of the ATP have to behave in a non-prejudiced manner.
"I just think there are umpires who are not behaving the way they should."
Asked if Davydenko was worried by the situation, Immenga added: "Yes, absolutely. If you see him playing and he does 10 double-faults, my guess is that everyone can observe the mental pressure is very hard.
"I really need to underline that everyone who has met Davydenko knows that he is a very funny, very nice, straightforward guy. He is not the kind of guy you would think of doing this kind of thing, and I would really put my hand into the fire for him."