Rehmann was a successful junior player, winning the Boys' Doubles at the 1993 Australian Open, with countryman Christian Tambue. They defeated the American pairing of Scott Humphries and Jimmy Jackson 6–7, 7–5, 6–2. In the same year he was also the junior single and doubles champion at Germany's national championship and also reached the final of the doubles at the 1993 Ansett Australian Indoor Championships. Along the way he and partner Alexander Mronz were victorious over top seeds Todd Woodbridge and Mark Woodforde.
The following year he competed in the singles of the 1995 Australian Open, the only other Grand Slam that he got to compete in. He defeated Luiz Mattar of Brazil 6–3, 6–4, 6–1 and make it into the second round, but then lost to eventual quarter finalist Andriy Medvedev 5–7, 4–6, 1–6. In Seoul he reached another ATP final but was defeated again, this time to Greg Rusedski.[2]