In 1949, Mercelis won the Girls' Singles of the
Wimbledon Championships. She competed every year at
Wimbledon between 1951 and 1968, and at the
French Open between 1952 and 1965. In the French Open, she reached the quarter-finals in 1957. Her main international career singles highlights included winning the
South of France Championships two time (1956–1957), the Cannes Carlton Club Championships (1957), the
Trophée Raquette d'Or (1957), the
Moscow International Indoor Championships two times (1958–1959), the
French Covered Court Championships (1958), the
German Covered Court Championships (1959), the
Belgian International Championships three times (1959–1960, 1964), the
Kent Championships (1960), the
Dutch International Championships and the Cannes Gallia Club Championships (1961), the
Le Touquet International Championship (1963), the
Knokke Le-Zoute International two times (1961, 1964). Mercelis played for
Belgium in the
Federation Cup from 1963 to 1964 and from 1966 to 1969, losing all five singles matches, and winning two of her eight doubles matches. She is the oldest player to have played for Belgium, at 37 years 231 days in her last doubles match against
South Africa on 24 May 1969, which she won partnering
Michele Kahn. In addition she also won the Belgian National Indoors Championships (closed to Belgian players only) 13 times consecutively (1956–1968) as well as, 14 women's doubles titles, and 16 mixed doubles titles, of which 13 were partnering
Jacky Brichant, and the
Belgian National Championships (closed) singles event 12 times ( 1951, 1953–1961, 1963–1964, 1966). ==Other titles==