 After regular club captain Stefan Kulovits suffered an injury which kept him sidelined for the entire first half of the season, Diekmeier played 17 matches as captain for Sandhausen.On 26 May 2020, Diekmeier finally scored his first ever professional goal at the age of 30 in Sandhausen's 1–0 away win over SV Wehen Wiesbaden, a feat which the club celebrated by offering free printing of his name and number on their replica kits.This goal proved momentous in marking the first of four consecutive victories for Sandhausen towards the end of the 2019–20 season, the points from which they secured to stay in the 2.