 Born in Castrop-Rauxel, North Rhine-Westphalia, Kruska started his career at SC Arminia Ickern and soon signed for VfR Rauxel 08, the team from his native village.In 1999, age 12, he was spotted by Borussia Dortmund and at the Westfalen Stadium he enjoyed his further training.Just 17, he made his debut in the first team against Kaiserslautern in 2004–05.