Krzewina appeared on the national junior scene in 2007, placing third at the national junior championships in the 400 metres. He dipped under 48 seconds in the event for the first time that year. He won his first international selection at the 2007 European Athletics Junior Championships as part of Poland's 4×400 metres relay team. The team (including Jan Ciepiela, Sebastian Porzadny, and Marcin Klaczanski) reached the final, but was disqualified. His first individual international race came the year after at the 2008 World Junior Championships in Athletics, but he failed to progress beyond the heats. He was again chosen for the relay and he helped a team of Arkadiusz Wojno, Michał Pietrzak, and Rafał Omelko take fifth in the final. The 2009 season marked a career progression as he won the Polish under-23 title with a personal best of 46.66 seconds. He also won a relay gold medal at the 2009 European Athletics U23 Championships, running alongside Marcin Sobiech, Pietrzak, and Ciepiela.