Set to battle it out in Bucharest, La Rojita - five-time winners of the tournament - and their Ukrainian counterparts drew a dead rubber 2-2 in the group stage, but the teams now reconvene with much more at stake.
Match preview
Though they had already cruised through to the last eight with wins in their first two matches, Spain's first encounter with Ukraine at this summer's finals required a last-minute Abel Ruiz equaliser to preserve their impressive unbeaten streak.
Now standing at a total of 14 matches, following a dramatic defeat of Switzerland last weekend, La Rojita's run has led them to a sixth Under-21 semi-final out of seven; equalling Italy's record of 11 overall.
They needed extra time, though, to overcome the Swiss in Saturday's quarter-final, when a deflected shot from Juan Miranda restored their lead after Zeki Amdouni had struck in stoppage time at the end of the second half.
Manchester City's Sergio Gomez had provided the opener, and was later named 'Player of the Match' by UEFA's technical panel, as for the first time in the tournament, Spain's mettle was fully tested.
By reaching the final four, Santi Denia's young charges have also assured a place at next year's Paris Olympics, but that feat will surely not satisfy their ambition: after losing to Iberian rivals Portugal in the semis two years ago, they are determined to regain the title their nation last claimed in 2019.
While Spain were Group B winners, finishing ahead of Ukraine on goal difference following the teams' draw last week, both beat Croatia and hosts Romania to secure a spot in the knockout phase - and the Ukrainians are also unbeaten so far this summer.
Many observers thought that run would end on Sunday, but Ruslan Rotan's side shocked favourites France in Cluj-Napoca, as star man Mykhailo Mudryk returned to the lineup and played his part in a 3-1 comeback win; with Shakhtar Donetsk midfielder Heorhiy Sudakov bagging a first-half brace.
As a result of France's elimination, Ukraine have also qualified for the Olympics, and with that achievement already surpassing most expectations they can now aim to break new ground by winning the Under-21 Euros for the very first time.
Runners-up in 2006, Ukraine have only once before made the final at this level - though they were memorably crowned Under-20 world champions four years ago - so Rotan's boys will be doubly determined to provide some rare good news for their troubled nation.
Spain Under-21s European Under-21 Championship form:
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Ukraine Under-21s European Under-21 Championship form:
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Team News
Spain named a full-strength starting XI against Switzerland, following their wholesale changes in the final group game, and a similar side should line up at Steaua Stadium.
Braga's Abel Ruiz will wear the captain's armband as he starts up front, supported by Real Betis winger Rodri, who only featured as a substitute against Ukraine last week. The latter was ranked second overall for both passes in the attacking third and take-ons during the group stage.
Second-half substitutes Aimar Oroz and Adrian Bernabe made a major impact against the Swiss, and either could come into Santi Denia's thinking should the Spain coach wish to freshen things up in midfield.
Meanwhile, midfielder Georgiy Sudakov is Ukraine's top scorer in the competition with three goals, and their quarter-final hero is set to start in an advanced role again.
However, Ruslan Rotan must make an enforced change in attack, as forward Dmytro Kryskiv serves a suspension for accumulated bookings; Bohdan Viunnyk, Vladyslav Vanat or Danylo Sikan will therefore lead the line up front.
Oleksiy Kashchuk plays a support role, having been - alongside Sikan - part of the squad which won the Under-20 World Cup in 2019.
Spain Under-21s possible starting lineup:
Tenas; V. Gomez, Pacheco, Paredes, Miranda; Blanco, Baena; Rodri, Sancet, S. Gomez; Ruiz
Ukraine Under-21s possible starting lineup:
Trubin; Sych, Batahov, Talovierov, Vivcharenko; Bondarenko, Brazhko; Kashchuk, Sudakov, Mudryk; Sikan
We say: Spain Under-21s 3-2 Ukraine Under-21s (after extra time)
Spain certainly start favourites, given their pedigree and recent form. However, Ukraine are no pushovers, having scored in each of their last 19 matches, losing on just two occasions.
Both teams are strong in the final third, so several goals may be traded - with an extra 30 minutes potentially required to separate them.