The moment Real Madrid lost their chances of retaining the title probably came before a ball had been kicked, when Cristiano Ronaldo pushed the referee during August's Spanish Super Cup first leg at Barcelona.
Ronaldo was shown a second yellow card late on in Madrid's 3-1 victory, after referee Ricardo De Burgos Bengoetxea ruled he had dived to try and win a penalty. The Portugal captain's angry reaction earned him an extra four-game suspension on top of the mandatory one-match ban for the sending off, meaning he sat out the first month of the 2017-18 La Liga season.
Madrid did not miss Ronaldo too badly in the second leg, as they completed a 5-1 aggregate victory which had Barca defender Gerard Pique admitting that for the first time in his career, he felt his side had been inferior to their Clasico rivals.
At that point few believed Ernesto Valverde's side, still reeling from Neymar's shock summer departure to Paris Saint Germain, would be able to get their house in order quickly enough to mount a real challenge for the title.
But they got a very welcome boost by how slowly Madrid began the campaign. With their best goal scorer watching on, Zidane's side drew their first two games at the Bernabeu, against Valencia and Levante. On Ronaldo's return, he was clearly well behind everyone in match-fitness, and Real Betis won 1-0 at the Bernabeu.
Meanwhile, Lionel Messi scored nine times in Barca's first five La Liga games, generating a momentum which pushed the team along all year. By mid-September Madrid were already seven points behind the runaway leaders, and minds were (consciously or not) focusing on a third successive Champions League title as the objective for this term.
By early November, Messi had found the net 12 times in Barca's first 11 games of the domestic campaign. Ronaldo had still scored just once in La Liga and was quite amazingly looking low on confidence in front of goal, not being helped at all by having had to play catch-up from day one.
He then made a bet against sceptical teammates that he could catch up in the Pichichi top scorers race, and has made a really impressive effort to do so.
However, Messi and Barca have remained ahead the whole way -- with the Catalan side's 3-0 Clasico win at the Bernabeu in December ending any realistic doubt over where the title was heading.
Zidane has said many times through recent months, especially as both Ronaldo and the team generally have picked up their form, that their issues at the start of the season were down to missing chances.
"At the start of season we always want to win everything," Zidane said in early April. "But sometimes, like this year, the reality is we have some moments of difficulties, above all in scoring goals. Personally, I am more excited and motivated by winning La Liga, it is the most difficult, and I believe that is also what the players want."
Ronaldo did click into form, maybe the best of his career, after the winter break. As well as firing Madrid past PSG and Juventus in the Champions League, he has scored in each of his last eight La Liga games -- 16 goals in total. Even after resting Ronaldo for some away trips, Zidane's side actually have a better record than Barca over the second half of the league season. But the damage had already long been done.
The flip side is that the slow start to the La Liga campaign has allowed Madrid to focus their energy on Europe. They also did not seem too bothered about exiting the Copa del Rey at home to neighbours Leganes back in January, with Ronaldo also sitting that one out.
Barca and Messi have barely rested throughout the season, which has seen them ease to a La Liga and Copa del Rey double, but arguably cost them in the Champions League when an astonishingly flat performance saw Valverde's side eliminated against Roma in the last eight.
Whether Madrid would have had the mental and physical resolve to push all the way in each competition is impossible to know -- but we never got a chance to find out, due to Ronaldo's moment of madness back before the season had properly begun.