This was no classic. With 12 minutes left, Real Madrid and Atletico were going through the motions and heading towards a dull draw in a game which had started late and was leaving little for the memory bank for fans of either team. But then, Angel Di Maria came on.
Left out of Madrid's Uefa Super Cup win over Sevilla last week, the Argentine winger was again on the bench for this one and there he stayed, even when Cristiano Ronaldo was withdrawn through injury at half-time.
Ronaldo was replaced by summer signing James Rodriguez, left out himself to accommodate a three-man midfield featuring the returning Xabi Alonso alongside Luka Modric and Toni Kroos. But the spark was missing.
The last time these two teams met, in the Champions League final in Lisbon last May, it was Di Maria's remarkable run from midfield that set up Gareth Bale for the decisive goal in extra-time after Sergio Ramos had headed home a dramatic equaliser right at the end.
With Ronaldo off, Bale below par and James still settling into his new surroundings, Madrid missed the Argentine's boundless energy in midfield on Tuesday, the threat he offers when running at rival defenders and his overall enthusiasm.
ADMIRER | Simeone knows Di Maria could hurt his team in Friday's second legWhen he came on, the spark immediately returned as a couple of dangerous runs unsettled the Atletico defence. And three minutes later, Madrid took the lead. Although not directly involved in James' strike, Di Maria's introduction had given the home side a significant psychological lift - and that led to the goal.
"The game changed with Di Maria," Atleti boss Diego Simeone said afterwards. "It's logical - he's the best player they have. He's the player most likely to create openings in the opposition half."
Mind games, perhaps, from the Argentine coach ahead of Friday's second leg, but whether he is right or not, one thing is certain - Madrid don't have another player like Di Maria.
Aware that the winger may yet leave against his wishes, Ancelotti chose his words carefully. "Di Maria is one of many good players," he replied. "But I think he forgot Cristiano, who won the Ballon d'Or and Bale, who scored in Lisbon. Di Maria is a Real Madrid player and I'll use him as I want."
Ancelotti wants Di Maria to stay and the Argentine is open to extending his spell at the Santiago Bernabeu - if the club can match his wage demands with a new contract this summer.
"Di Maria knows what it is to be at Real Madrid and he knows what it is to be elsewhere. Let him weigh things up and decide if he wants to leave or not," Sergio Ramos said in the mixed zone after the game. "But this is a club where you want to be for your whole life, so I don't think he wants to leave."
The winger's importance increased dramatically in 2013-14. Previously inconsistent and often frustrating, Di Maria impressed Ancelotti after a similar transfer tangle last summer and stayed at the club as the Italian opted to dispense with Mesut Ozil instead.
The 26-year-old initially found himself on the bench, but became a key player as Ancelotti altered Madrid's formation and featured in a midfield position which saw him double up as an extra attacker - almost like two players in one. It worked wonderfully.
The Argentine went on to score in the final of the Copa del Rey as Real beat Barcelona to that trophy and also set up Bale for the decisive goal against Atletico in Lisbon as Madrid won La Decima. Without Di Maria, it may never have happened.
So while Kroos looks like a superb signing in midfield and James will surely blossom into one of the world's finest footballers, Madrid need strength in depth this term as they play for La Liga, the Copa del Rey, the Club World Cup and attempt to become the first team to retain the Champions League since the format of that competition changed from the old European Cup in 1993.
With James settling in and Ronaldo still struggling with injuries, Di Maria well could hold the key to Madrid's success this season - just like he did last year. He simply must be kept.