Jemelka was one of five players promoted from Sigma Olomouc's B-team to the senior team ahead of the 2016–17 season, which the club played in the second-tier Czech National Football League having been relegated the previous season.[3] He made his Czech First League debut on 30 July 2017 in a match against Mladá Boleslav.[4] His first goal came in a controversial manner in Sigma's 1–1 draw at Slavia Prague on 10 September 2017, scoring the goal with his hand.[5] After playing the first six games of the 2020–21 season, Jemelka joined Belgian Pro League side OH Leuven on loan for the rest of the season in October 2020.[6]
Jemelka joined Viktoria Plzeň in August 2022, signing a three-year contract with the club.[4][7] His first competitive goal for Plzeň came against Mladá Boleslav; Jemelka scored the second goal in a 2–0 Czech First League win in October 2022.[8]