The 20-year-old world No.29 was shown on an Instagram story posted, and later deleted, by two-time Slam champion Svetlana Kuznetsova from a chartered flight to Melbourne.
The clip surprised tennis fans given Yastremska is on a provisional suspension from the World Anti-Doping Agency after a November urine sample found the presence of mesterolone metabolite, an anabolic agent on the ITF Prohibited List.
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Yastremska has the right to play but only pending an appeal at an official hearing, which the ITF said last week she was yet to make.
Former Australian anti-doping chief Richard Ings questioned whether Yastremska’s presence on the flight to Melbourne, which was paid for by Tennis Australia, broke WADA rules.
Making matters worse the social media footage showed intermittent mask use.
Meanwhile Ash Barty is hoping to gain an advantage in the lead-up to the Australian Open as she bases herself in Melbourne while her rivals are stuck in hotel quarantine.
The world No.1 travelled from Brisbane to Victoria before movement between the two areas was restricted, and has practiced at Rod Laver Arena before it was locked down.
International tennis stars will be allowed to train for short periods at the Melbourne Park precinct but will spend around 19 hours a day in their hotel rooms, while Barty and other locals live freely.
Barty is keen on playing in the Adelaide exhibition event scheduled for later this month, which will feature top players like Serena Williams, Rafael Nadal and Novak Djokovic.
All players will be out of quarantine in time for five lead-up tournaments in Melbourne in the week before the Australian Open - two women’s events, two men’s events and the ATP Cup.
Several players have not travelled to Australia due to positive COVID-19 tests, including Andy Murray and Madison Keys.