World No. 1 and reigning French Open champion Ash Barty announced that she will not be defending her title later this month in a statement on Instagram.
Barty, of Australia, cited concerns about COVID-19 and a lack of proper preparation time.
"It has been a difficult decision to make, but unfortunately, I will not be competing in Europe this year," she wrote. "Last year's French Open was the most special tournament of my career, so this is not a decision I have made lightly.
"There are two reasons for my decision. The first is the health risks that still exist with COVID. The second is my preparation, which has not been ideal without my coach being able to train with me due to the state border closures in Australia."
The 24-year old Barty, who defeated Marketa Vondrousova in Paris for her maiden Slam title, chose to not play at the US Open, which is currently underway in New York.
Barty said she will not play again this year and will turn her focus to the 2021 season.
"I now look forward to a long preseason and the summer in Australia," she wrote. "It has been a challenging year for everyone, and although I am disappointed on a tennis front, the health and well-being of my family and my team will always be my priority. Thank you to my fans for your continued support, I can't wait to play for you again."
Barty had a career-best season in 2019, winning the French Open and the year-end WTA Finals and earning titles in Miami and Birmingham. She took over the top ranking for the first time in June 2019.
She reached the semifinals at the Australian Open to start the year before losing to eventual champion Sofia Kenin. She most recently played in February at the Qatar Open.