Hungary reached next summer's tournament with a 2-1 win over Iceland, while Serbia's dreams were shattered as they were beaten on penalties by Scotland after drawing 1-1 in 120 minutes.
Hungary produced a stunning comeback to beat Iceland as they trailed from the 11th minute but equalised through Loic Nego with two minutes of the 90 left, before Dominik Szoboszlai burst through the heart of the Icelandic defence and curled in the match-winner in the second minute of injury time.
Marco Rossi's side will now join Portugal, France and Germany in the 'group of death' at Euro 2020, with two of their games set to be played in Budapest.
For now though, their attentions will turn to the Nations League, as they can still win their group and earn promotion to League A.
Hungary, ranked 47th in the world, are currently a point behind group leaders Russia, the only team they have lost to in four games so far.
Serbia – ranked 30th – followed that defeat up with a 2-2 draw with Turkey, and are now fighting to avoid relegation back to League C.
They are bottom of the group, having not won any of their first four games. They have scored just three goals, with Aleksandar Mitrovic netting two of those.
Mitrovic missed the decisive penalty in their shootout defeat to Scotland after Luka Jovic had equalised late in normal time to keep their hopes alive.
It means that Serbia have now failed to qualify for the European Championships in each of their four attempts since first competing as an independent country in 2006.
The Eagles have not kept a clean sheet in their last eight away matches, last doing so in Romania in October 2018 – before Ljubisa Tumbakovic was in charge.
Hungary Nations League form: WLWD
Hungary form (all competitions): WLWWDW
Serbia Nations League form: LDLD
Serbia form (all competitions): LDWLDD
Loic Nego may have put himself in the frame to start for Hungary after coming off the bench to score against Iceland.
Marco Rossi may also bring in the likes of David Siger and Nemanja Nikolic as he aims to keep his side fresh ahead of their final Nations League game against Turkey on Wednesday.
After not featuring against Scotland, Serbia captain Aleksandar Kolarov could start, but his club Inter Milan are keen for him not to play a part as he has been suffering from a muscle injury lately.
The attacking trio of Dusan Tadic, Aleksandar Mitrovic and Luka Jovic have not all been deployed together in the Nations League so far, but could all start as Serbia search for goals.
Hungary possible starting lineup:
Gulacsi; Orban, Lang, Szalai; Nego, Siger, Nagy, Szoboszlai, Holender; Szalai, Nikolic
Serbia possible starting lineup:
Dmitrovic; Milenkovic, Spajic, Kolarov; Gudelj; Lazovic, Milinkovic-Savic, Tadic, Mladenovic; Jovic, Mitrovic
We say: Hungary 1-0 Serbia
This is unlikely to be a classic, given that both teams will still be recovering from their gruelling Euro 2020 playoff games. Morale will also be low in the Serbia camp following their defeat, which could see Hungary sneak a one-goal victory.