After having given up his studies for building industry technology at the Verkehrshochschule in Dresden, he underwent training as an air traffic controller. Then he was trained in speaking as well as presenting for radio at the Berliner Rundfunk for two years.
1988 he passed an application test for a position as a broadcast news analyst for the DFF. After that he worked for the youth radio DT 64. After speaking for radio dramas, features, documentaries and commercials between 1988 and 1991, the former chief speaker Werner Veigel took him to the Tagesschau in 1991 - initially Riewa was heard there as a voice actor, followed by a position as a broadcast news analyst from 1994 onwards. Since 6 September 1995, he has been the presenter of the main edition of the Tagesschau at 8 pm.
In 1994 Dieter Thomas Heck discovered him as a television presenter for the Deutsche Schlagerparade, after Birgit Schrowange had given up presenting the broadcast after only one year. Riewa also presented the German qualifications for the Eurovision Song Contest in 1996 and 1997.
In 1998, after recurring rumours, he stated that he was not gay. He succeeded in suing two publishers for 15.000 DM compensation after they had proclaimed him as homosexual. He had an affair with German singer Michelle in 2002. His confessions about sexual intercourse with her annoyed his employer ARD, which he then apologised for, so it did not lead to any consequences. In 2004 the ERC Ingolstadt-fan Riewa shouted at a German ice hockey game: "Scheiße, Scheiße, Scheiß Iserlohn." commenting on the team of Iserlohn Roosters which earned him even more critical remarks from his employer.