Rietmann joined FC Basel's first team for their 1900–01 season. Rietmann played his domestic league debut for the club in the away game on 25 November 1900 as Basel played a 3–3 draw with FC Zürich.[1]
For the team the 1900–01 season was a bad season, they ended the group stage in fifth position. A curiosity in this season was the away game on 3 March 1901. This was an away game against Grasshopper Club and it ended in a 3–13 defeat. The reasons for this high defeat can be explained with the fact that one of the players missed the train and that the team played with a number of players from their reserve team. Nevertheless, to date this remains the teams’ highest and biggest defeat in the club's history.[3]
During the following two season Rietmann played mainly for Basel's second team who played in the Serie B, the second tier of Swiss football.[4]
He again played for the first team in 1904, an away game in France against FC Mulhouse which ended in a high scoring 6–8 defeat. Rietmann was amongst the goal scorers.[5] Between the years 1900 and 1904 Rietmann played at least five games for Basel.[note 03–04 1] One of these games were in the Swiss Serie A and the others were friendly games.[6]