On the eve of his return to the court, Novak Djokovic has weighed in on tennis´s high-profile doping cases and criticized what he perceives as double standards in the sport. The former world No. 1, who is chasing a record 25th Grand Slam title at next month´s Australian Open, expressed his frustration Sunday at being “kept in the dark” regarding top-ranked Jannik Sinner´s doping case. Djokovic is the top seed in the Brisbane International this week, making his first appearance at the event since 2009. He will also team up with Australia´s Nick Kyrgios in doubles, with the duo headlining Monday´s action. Off the court, Djokovic lent his voice Sunday to Kyrgios´s strong criticisms made a day earlier, speaking out at length about the ongoing doping case involving current world No.1 Jannik Sinner. “I´m not questioning whether (Sinner) took the banned substance intentionally or not,” Djokovic said at a press conference Sunday in Brisbane. “We´ve had plenty of players in the past and currently under suspension for not even testing positive to banned substances. “Some players with lower rankings waiting for their case to be resolved for over a year. I´ve been really frustrated … to see we´ve been kept in the dark for at least five months ( on the Sinner case ).”