Alexander Povetkin, Anthony Joshua's proposed opponent, has signed up for a Voluntary Anti-Doping Association programme.
Povetkin's team are closing in on a deal for the world heavyweight fight after the WBA ordered Joshua to sign for a mandatory title defence against the Russian.
The 38-year-old failed two drug tests in 2016 but has opted to join the VADA plan with the organisation confirming the move on Twitter on Sunday.
Povetkin is set to receive the September fight after WBC champion Deontay Wilder was unable to agree terms for a unification clash against the British fighter.
WBA president Gilberto Mendoza had granted Joshua an extra month to continue talks for a fight with Wilder, but with a deal yet to be agreed, he has called on AJ to finalise a bout with mandatory challenger Povetkin.
Following the WBA's instructions, Joshua's promoter Eddie Hearn admitted: "If it is Povetkin, it will be in September, that's for sure, and then we fight Wilder straight after if he signs a contract."