Complex sentence: Katsivelis started his career playing at the semi-professional level (the 3rd-tier level of Greek basketball) with Mantoulidis, and he signed with the Greek League club Olympiacos Piraeus in 2010.
Split into simpler sentences: Katsivelis started his career playing at the semi-professional level (the 3rd-tier level of Greek basketball) with Mantoulidis. He began his professional career when he signed with the Greek League club Olympiacos Piraeus in 2010.