Krzewina missed most of the 2010 season but returned in 2011 and made his first impact at the senior level. He anchored the Polish 4×400 m relay team at the 2011 European Athletics Indoor Championships, taking the team to fifth place. The team attempted to defend their title at the 2011 European Athletics U23 Championships, but were pipped to the gold by a tenth of a second by the British team.[5] At the Polish Athletics Championships he placed fourth in the 400 m with a new best of 46.51 seconds.[4] This earned his global senior debut at the 2011 World Championships in Athletics,[7] 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.[5]
He began 2012 in peak form, setting an indoor best of 46.85 seconds then winning his first national title at the Polish Indoor Athletics Championships.[8] 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.[5] He performances declined further in 2013, having only a seventh place finish nationally.[4]
Krzewina improved at the start of 2014 with an indoor best of 46.54 seconds to finish as runner-up at the national championships. He anchored a team of Kozłowski, Omelko and Pietrzak to take Poland to fourth place in the 4×400 m relay final at the 2014 IAAF World Indoor Championships on home turf in Sopot (Krzewina's highest global finish at that point). He opened the outdoor season with a big personal best of 45.11 seconds.[9] This marked ranked him fourth in the European rankings for that season.[10] He repeated that time at the Polish Championships to claim his first national outdoor title in the 400 metres.[4] He was chosen for both the individual and relay events at the 2014 European Athletics Championships. In his solo senior debut for Poland he reached the 400 m final and finished in fourth place, just behind Israel's Donald Sanford.[11] 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).[12] 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.[13]
The Polish quartet of Karol Zalewski, Rafał Omelko, Łukasz Krawczuk and Krzewina broke the world indoor record in the men's 4x400m 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.[14]
