Kleckers first drew attention in 2013 when, at the age of 17, he captured the highest ranking and most prestigious amateur event in Germany by defeating Roman Dietzel 4–2 in the final of the German Amateur Championship. In the next few years he twice played in the qualifying rounds for the World Championship, losing 10–6 to Noppon Saengkham in 2015 and 10–7 to Rory McLeod in 2016. At the 2015 Riga Open he won a match in a European Tour event for the first time by beating Anthony Hamilton 4–0, before losing 4–0 to Stephen Maguire.