Prajoux became a professional tennis player in 1972. In singles he could not celebrate any major successes on the ATP Tour , he was more successful in doubles. In the course of his career he was able to win six double titles, including the Masters title in Rome in 1978. His highest ranking in the tennis world rankings he reached in 1976 with position 66 in singles and in 1982 with position 17 in doubles.
His best individual result in a Grand Slam tournament was reaching the third round at the French Open in 1975, 1976 and 1980. In the doubles competition, he stood alongside his compatriot Hans Gildemeister in the final of the French Open in 1982 . They were defeated by number one Sherwood Stewart and Ferdi Faygan from the USA . He was also in 1980 with Francisco González in the round of 16 of the Wimbledon Championships and in 1975 with Ricardo Cano in the quarter-finals of the US Open . In mixed he reached the last sixteen of the French Open three times.
Prajoux played ten singles and 13 doubles games for the Chilean Davis Cup team between 1972 and 1984 . His balance was positive, 6: 4 in singles and 8: 5 in doubles. The highlight of his Davis Cup career was his participation in the 1976 world group final against Italy . Chile lost 4-1, while Prajoux took the only point in his individual match against Antonio Zugarelli .