Catherine Cartan "CiCi" Bellis (born April 8, 1999) is an American professional tennis player. In early 2018, she was the second youngest player ranked in the Top 100 by the Women's Tennis Association (WTA) and has a career-high ranking of No. 35 in the world, which she achieved in August 2017. Bellis is known for winning a match at the 2014 US Open as a 15 year old against a Top 20 opponent, making her the youngest match winner at the US Open since 1996.
Bellis's biggest professional title came at the Hawaii Tennis Open in 2016, a WTA 125K event. She has also won six singles titles and two doubles titles on the ITF pro circuit. Bellis had an accomplished junior season the same year she played in her first US Open, finishing the 2014 season as the ITF Junior World Champion for holding the year-end world No. 1 junior ranking. She also won the USTA National Junior Championship that year at 15 to become the youngest winner of the event since Lindsay Davenport in 1991.