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. where the Polish team of
Kacper Kozłowski,
Piotr Wiaderek, Krzewina and
Marcin Marciniszyn ran in the relay. The team finished their heat in 3:01.84 minutes (the fastest for the country that year and of Krzewina's career) but this was not enough to make the final. An appearance at the
2012 IAAF World Indoor Championships followed but the team faltered in the
relay final and came in sixth (last) place. He achieved a season's best of 46.55 seconds outdoors, but did not compete internationally as a poor performance at the national championships meant he was excluded from the relay team that
competed at the 2012 Olympics. This marked ranked him fourth in the European rankings for that season. He repeated that time at the Polish Championships to claim his first national outdoor title in the 400 metres. Having missed the heats, he was drafted in as the last man for the Polish team in the
4 × 400 m relay final. Starting his leg in fourth place, he narrowly overhauled
Thomas Jordier in the final straight to win the bronze medals for Poland (setting a personal relay best of 2:59.85 minutes in the process). His fourth in the individual event led to his inclusion on the European relay team for the
2014 IAAF Continental Cup and (teaming up with European rivals
Conrad Williams, Sanford, and
Martyn Rooney) he helped the team to silver behind the African team. Krzewina and his teammates qualified to the
4 × 400 metres relay final at the
2016 Summer Olympics in
Rio de Janeiro, Brazil. The Polish quartet of
Karol Zalewski,
Rafał Omelko,
Łukasz Krawczuk and Krzewina broke the world indoor record in the men's 4 × 400 m with a stunning finish to the final track event of the
2018 World Indoor Championships in Birmingham. On the last straight, Krzewina overtook the anchor of the American team, which lead from the start, and achieved the greatest success in his career. In October 2022, Krzewina was issued with a 15-month ban backdated set to expire in December 2023 for an anti-doping rule violation for whereabouts failures. ==Competition record==