Mestach, who started playing tennis at the age of four, prefers grass courts. She played her first ITF tournament in 2008 in Koksijde , where she was eliminated in the first round. In May 2009 she made it to the semi-finals at the tournament in Antalya , and reached the quarter-finals in Bree . A year later she moved into the final in Tortosa , which she lost to Victoria Larrière in just three sets. Another finals followed in Balikpapan in August . At the first ever Youth Olympic Summer Games , she won the bronze medal in the double competition together with Tímea Babos in 2010, seeded number one . In the individual, she was eliminated as number five on the seeding list in the round of 16.
In 2011 Mestach won the Australian Open for Juniors. Seeded in second place, she won the final against the five seeded Mónica Puig from Puerto Rico 6: 4, 6: 2 and thus became the new number one of the juniors. She also won the final in doubles ; with Demi Schuurs they defeated the Japanese pairing Eri Hozumi and Miyu Katō 6: 2, 6: 3. In the course of the year she played increasingly high-value ITF tournaments; she reached the quarter-finals at the $ 25,000 tournament in Tessenderlo . In May she made her debut on the WTA Tour in Brussels , where she failed in round one with 2: 6, 6: 7 to Varvara Lepchenko .
On September 14, 2015, she reached a new personal record in the WTA world rankings with 98th place in the individual; in doubles she was on position 64 on February 1, 2016. From 2010 An-Sophie Mestach played for the Belgian Fed Cup team (record: 8 wins, 6 defeats). She ended her career in early 2019.