In 1970 Semmler together with Andreas Decker in four in the Spartakiade and at the Youth World Championships. 1974 Semmler was with the aft GDR champion and then European champions. In 1974, he joined the coxless four. In the occupation Siegfried Brietzke, Andreas Decker, Stefan Semmler and Wolfgang skimmed the boat from SC DHfK Leipzig dominated over the next few years, this class of boat. 1974 in Lucerne and 1975 in Nottingham, the four rowers were world champions, 1976 Montreal Olympic champion. 1977 in Amsterdam and 1979 in Bled, the boat was world champion again, only in 1978 in New Zealand succumbed to the Soviet boat and won silver. In the Olympic season 1980 skimmed injured his hand and was represented by Jürgen Thiele. Together, the crew won at the Olympic Games in Moscow well before the boat of the hosts.
Semmler was a trained mechanic telecommunications, studied at the DHfK and was later Assistant in water sports. When MC Zschopau he oversaw the Six -Days Team of the GDR.