Polish world number 77 Kamil Majchrzak said Thursday he had failed a drugs test, describing it as "the biggest shock."
The 26-year-old explained he tested positive "in anti-doping controls during October and November".
"This is the biggest shock. I have started the most difficult fight of my life - the fight to prove my innocence."
Majchrzak, who reached a career high in the rankings of 75 in February, made the semi-finals in Pune, India, this year as well as two quarter-finals in Geneva and Sofia.
His best run at the Grand Slams came in 2019 when he went to the third round of the US Open after making it through qualifying.