Croatia Under 21 and Hamburg defender Mario Vuskovic began a challenge against his ban for taking blood booster EPO at the Court of Arbitration for Sport on Tuesday.
The 22-year-old, who was handed a two-year suspension last year, says he is innocent.
However in a cross appeal, the German Anti Doping Agency and the World Anti Doping Agency say the two-year ban – imposed by the German Football Association – should be increased to four years.
Having begun his career at Hadjuk Split, Vuskovic joined Hamburg in 2021, initially on loan before signing a permanent deal.
He failed a drug test in November 2022.
