Jan 1 (Reuters) - U.S. Open champion Emma Raducanu has withdrawn from this week"s Australian Open warm-up tournament in Melbourne having just come out of isolation after testing positive for COVID-19, she said on Saturday. Raducanu, who became the first qualifier to win a Grand Slam when she triumphed at Flushing Meadows in September, had pulled out of the Mubadala World Tennis Championship exhibition event in Abu Dhabi last month after contracting the virus. The 19-year-old was scheduled to headline one of the two women"s WTA 250 warm-up tournaments taking place at Melbourne Park -- dubbed the "Melbourne Summer Set" -- from Jan. 4-9. "The timing to compete in the first Melbourne event this week is too soon for me, having just returned from isolation," Raducanu was quoted as saying by the organisers. The world number 19 is scheduled to compete in the Sydney Classic later this month before making her debut in the Jan. 17-30 Australian Open.