Brussels: Kim Clijsters, the former world No.1 who made her comeback this week in Dubai, had to undergo six doping tests before she could return to the circuit, according to official data.

According to International Tennis Federation (ITF) documents published this week, the Belgian, who announced her comeback last summer, was obliged to take the out-of-competition tests to be able to return in 2020 as a 'reinstated' player.

Three-time US Open winner Clijsters, 36, had been out of the game for seven years before losing her return match 6-2, 7-6 (8-6) to world number 16 Garbine Muguruza, herself a two-time Grand Slam champion and finalist at this year's Australian Open.