With both Barcelona and Manchester United protecting 2-0 leads, a rematch of the 2009 Champions League final is looming at Wembley Stadium later this month.
But ensuring they each have a chance to win a fourth European title will require the semifinalists to successfully negotiate second leg matches that are shaping up to be contrasting occasions.
On Tuesday, Barcelona will be hosting a Real Madrid side - and particularly coach Jose Mourinho - still seething following an ill-tempered loss at home last week.
A raging Mourinho suggested there is a conspiracy to help Barcelona go through - one of the inflammatory moments last Wednesday that led to Madrid being hit with six UEFA charges.
Barcelona, though, is also under investigation after an angry scuffle between players and officials from both teams at halftime resulted in substitute goalkeeper Jose Pinto being sent off.
It means the repercussions from the first leg that scarred the reputations of both sides could still be felt long after the final whistle goes at the Camp Nou.
But Madrid has already started stoking the tensions as it tries to overturn a 2-0 deficit and enable Mourinho to reach a second successive final, having triumphed with Inter Milan against Bayern Munich last May.
Madrid has posted a video on its website calling on Belgian referee Frank de Bleeckere to avoid "falling for Barcelona's theater."
"We believe we can do it," midfielder Kaka said. "Madrid can never stop fighting."
Barcelona, whose lead was secured by Lionel Messi's double, is focused on the health of its injury-ravaged defense.
Center back Gabriel Milito tore a muscle in his left leg in Saturday's 2-1 defeat to Real Sociedad to join an injury list that already includes Adriano and Eric Abidal, although left back Maxwell could return on Tuesday.
That loss to Real Sociedad was Barcelona's first in the league in seven months. But with four games to go, Pep Guardiola's side still enjoys an eight-point lead over Madrid, which also lost to Zaragoza having rested several players.
Seeing a 31-match unbeaten run should serve as a "lesson" to his players, Guardiola said.
"Like any team we can win or lose - we were coming off a complicated situation," the Barcelona coach added.
Losing, though, in the Champions League is something that rarely happens after first winning away.
Only twice have teams overturned first-leg losses at home: Inter Milan earlier this season against Bayern Munich and Ajax against Panathinaikos in 1996.
Manchester United, which lost to Barcelona in the 2009 final, is certainly confident that Schalke's longest run in the competition will end at Old Trafford after its 2-0 collapse at home.
Key players including striker Wayne Rooney are set to be rested for Sunday's Premier League match against Chelsea, which is just three points behind United with three matches to go after the leaders lost at Arsenal on Sunday.
"I will make some changes on Wednesday without question," United manager Alex Ferguson said. "I will bring Paul Scholes back in, Dimitar Berbatov, Michael Owen.
"We'll need to do that but I will probably keep my experienced players at the back. We have a big week ahead. Wednesday is a European semifinal second leg and then the game on Sunday. Two massive games, so we will regroup and freshen it up."
History does not favor United on Wednesday, with the three-time European champions losing all four previous knockout phase matches against German sides.
Recent form, though, provides more encouragement: Schalke has lost three successive matches and was routed 4-1 by Bayern Munich on Saturday.
"It's never good to lose, but in principle we have nothing to play for in the Bundesliga," midfielder Alex Baumjohann said. "We can't hide our heads in the sand, we have to repeat the performances we had before. You always believe, as long as it's not finally decided (against United).
"But a lot of things have to come together for us to achieve a miracle. But if we don't believe in it, we might as well not fly to England."
Their chances, though, of returning to England for the May 28 final at Wembley are slim.
