Atletico Madrid will go eight points clear of Real Madrid if they beat them on Sunday, and they will still have a game in hand.
If Zinedine Zidane's team win however, the gap becomes two points and the pressure on Atleti to win that extra-match increases.
As the Spanish capital derby approaches, Sportsmail looks at the five areas where the potential title decider could be won or lost.
THE COACHES
Zidane had a tactical nightmare last Monday against Real Sociedad. His team had dominated the first half but at the restart he switched from 4-3-3 to 5-3-2, putting Casemiro into the middle of a back three.
The upshot was that David Silva, who had been anonymous in the first period, suddenly found space in midfield and began to dominate the game. Real Sociedad scored and Zidane immediately reverted to a back four.
'You have asked me three times about it so it could be the case,' he said in a spiky post-match interview when asked about if the tinkering had cost Madrid two points.
It was a rare blip in his coaching copybook. Earlier in the season when Real Madrid beat Atletico 2-0 he out-thought Diego Simeone, who took all the criticism after the game for having started with Hector Herrera, Felipe and Yannick Carrasco and then taken them all off at half-time.
He was also criticised for hooking Joao Felix when Atletico needed the goals to get them back into the game.
The battle of the coaches will be fascinating and decisive on Sunday.
JOAO FELIX OR VINICIUS?
They both feel they should be playing more, they both feel they should start on Sunday, and they both scored last week.
When Joao Felix hit the back of the net from the edge of the area in the victory over Villarreal he had plenty to say.
It was unclear who his rant – before putting his shushing index finger over his lips – was meant for but most guessed it was for his coach.
After the game team-mate Renan Lodi claimed the reaction had been aimed at him but no one in Madrid is really buying that.
It's clear Simeone and Joao do not see eye to eye but the Atletico coach doesn't care about the Portuguese not being a fan of his manager – he just wants him delivering and if he's angry and that helps him deliver then all the better.
Vinicius did not protest after his deflected goal gave Real Madrid a point last Monday.
Both will have to sweat on making the starting line-up. Both have the potential to be match winners if they do.
MARCOS LLORENTE - ZIDANE'S WORST NIGHTMARE?
Llorente is a player Zidane only ever saw as a reserve holding midfielder when he was at Real Madrid.
That is why he sanctioned his departure to Atletico at the start of last season. To say Simeone saw more in him would be an understatement of substantial proportions.
Released to play further forward Llorente is now a Spain international and Atletico's second top scorer with eight goals – one more than Joao Felix.
Llorente's ability to play anywhere sometimes counts against him and he was criminally misused against Chelsea in the Champions League last-16 first leg in Bucharest when he was asked to chug up and down the right as a wing back.
Simeone seems to have remembered just how devastating Llorente can be on the shoulder of the last defender and there is a good chance he will play up front with Suarez on Sunday.
If he runs amok it will be another uncomfortable reminder of what Real Madrid have lost.
TRIPPIER BACK JUST IN TIME
Atletico Madrid managed just one clean sheet during Trippier's absence and the end of his FA ban could not come soon enough for Simeone who had pretty much changed his system to bring the best out of the England international and then found himself without him.
When the club failed to lift the ban on appeal they did at least get permission for him to continue training. If he can come back fit and flying Atletico have a huge and very timely boost.
Former Atletico and Chelsea full back Filipe Luis is the perfect judge of this position and he told Sportsmail in a recent interview: 'Trippier is a full back who is very good at overlapping and getting to the opponent's area and crossing the ball.
'He is very calm on the ball. And he understands the system perfectly. He is playing at a very high level, one of the five best in the world, and he has the capacity to develop even more.'
Sunday will be the perfect stage to justify that praise.
THE PRECEDENT AND THE HEAD-TO-HEAD
Goal difference in LaLiga is head-to-head and that is currently bad news for Atletico Madrid because they were beaten 2-0 earlier in the season by Real Madrid.
If they lose this one not only will they be pegged back to two points but if they then finish the season level on points with Real Madrid, it will be Zidane's team who finish higher courtesy of the head-to-head goal difference.
That 2-0 win was also a humbling experience for Simeone. His team were outplayed all over the pitch.
We will find out on Sunday if it serves as a motivation for revenge or a precedent for the two teams' second duel of the campaign.