Tennis chiefs have been accused of sporadically giving advanced warning to players about anti-doping tests.
A report in British newspaper the Mail on Sunday said the governing International Tennis Federation (ITF) had allowed players to book timeslots before certain tournaments to provide a blood sample for the athlete biological passport (ABP).
Critics say this gives dopers the chance to hide their cheating.
"I don't think the ITF should announce when it will conduct doping tests on athletes," Luis Horta, the former head of Portugal's anti-doping agency, was quoted as saying by the Mail.
The ITF's anti-doping programme is run by the International Tennis Integrity Agency (ITIA), an independent body.
"The aim is to gather data from as many players as possible so we have the widest set of data to work from," the ITIA said in a statement.
"Logistically, it makes sense therefore to arrange this in advance once or twice a year, so we can test as many players as we can.
"Because we do this ABP testing on an ongoing basis – both with notice and no-notice – it does not make any difference if players know about it in advance. Adverse levels will show, either with this test or through in-competition or out-of-competition testing," the statement said.
