Mortensen became a professional tennis player in 1984. His first year on the ATP World Tour should also be his most successful. At the side of Jan Gunnarsson , he won the tournaments in Nice , Båstad and ATP Toulouse , and he won with Mats Wilander in Geneva . In the following years he was in several finals, including at the side of Mansour Bahrami at the Masters tournament in Monte Carlo but could not win any of them. It was not until 1989 that he achieved his fifth and last double title, this time together with Eric Jelen in Lyon . He reached his highest ranking in the tennis world rankings in 1984 with position 301 in singles and in 1988 position 34 in doubles.
In the individual, he could never qualify for a Grand Slam tournament. His best double result was the quarter-finals at the 1985 US Open , where he lost to the eventual tournament winners Ken Flach and Robert Seguso from the United States on the side of Hans Simonsson .
Mortensen completed 26 singles and 24 doubles games for the Danish Davis Cup team between 1979 and 1990 . His greatest success with the team was taking part in the quarter-finals of the world group. In the 0-5 defeat against Germany he lost with Michael Tauson against Boris Becker and Eric Jelen in three sets.
Denmark
Nice
Jan Gunnarsson
Hans Gildemeister Andrés Gómez
Juan Avendaño Fernando Roese
Geneva
Libor Pimek Tomáš Šmíd
Eric Jelen