Westphal began at the age of nine with the tennis game and won the 1980 U-16 European Junior Championships. In 1981, he won the individual title of the Challenger tournament in Travemünde. In 1984 he reached the final of the ATP tournament from Livingston and in the semi-finals of Munich and Toulouse. The following year he reached the final of Kitzbühel. In 1989, he was in both the single and double - on the side of Markus Zoecke - in the quarter-finals of Hamburg. His best finish in the tennis world ranking in 1986 was 49th in singles and 1989 space 293 in doubles.
He became known to a wider public in Germany through its stakes in the German Davis Cup team, of which he was between 1982 and 1986 and for which he played 19 singles matches. His greatest success with the team was the final attendance 1985 In the semifinals, he won as underdogs against Tomáš Šmíd from Czechoslovakia in a five-set match with 6:8, 1:6, 7:5, 11:9 and 17.: 15; Germany eventually won 5-0.
According to him, Michael Westphal Award is named, which is awarded to persons who have rendered outstanding services to the sport of tennis. His longtime companion was the actress Jessica Stockmann, who was later married to Michael Stich.
Westphal died on the night of June 20th 1991 from AIDS. He was buried at the cemetery Ohlsdorfer in Hamburg.