Bellis has won three singles and two doubles titles on the International Tennis Federation tour in her career. Bellis was ranked the number one junior tennis player in the world in September 2014, and was declared 2014 International Tennis Federation Junior World Champion. In August 2014, Bellis won the USTA Girls 18"s National Championships, defeating Tornado Alicia Black in straight sets to secure a main draw wild card for the 2014 United States Open.
She was the youngest winner at 15 years, 4 months of that event since Lindsay Davenport in 1991.
On August 26, 2014, Bellis defeated that year"s Australian Open runner-up Dominika Cibulková (ranked 13th at the time) at the 2014 United States Open, becoming the youngest player to win a match at the United States Open since Anna Kournikova won aged 15 in 1996. She was the youngest American to win a match at the United States Open since Mary Joe Fernández in 1986, the youngest woman in the main draw of a Grand Slam since Alizé Cornet at the 2005 French Open and youngest in the main draw of the United States Open since 2004.
Bellis was unable to win a second match, however, losing in three sets to the 20-year-old Kazakhstani Zarina Diyas in round two.