Defeat at Espanyol last time out leaves Madrid with a huge task to catch Barca over the 12 remaining games, although on Thursday Ernesto Valverde's men were held to a 1-1 draw at lowly Las Palmas.
Atletico Madrid are putting pressure on Barca, Diego Simeone's side taking 37 points from the last 42 on offer to cut the gap to five ahead of their trip to Camp Nou on Sunday.
Madrid have a chance to move closer to the top two with a home game against Getafe on Saturday, with Zidane still believing Los Blancos can defend their title.
"What we have to do is win games, that's all," Zidane told a pre-match news conference on Friday. "Keep going to the end of the season. That's what we have to do and that's what we have to think about.
"It's not easy to go out and win all the games but it's what the players have to try and do. If we're always motivated and thinking about the game, then we can achieve something great.
"There's always a league - it's right to the end for every team. Perhaps everyone thinks the league is over but in football you can never give up. That's what we have to do. We have to get it into our heads that our remaining league games are important.
"We're going to fight until the end of the league. It's complicated but it's not impossible, that's the difference between what I say and what you [the media] say. It's difficult, it's very difficult, but it's not impossible.
"We went on a run of five wins, that's football. Last Tuesday against Espanyol, we were angry to lose but we didn't expect to lose. We made some mistakes. When things go wrong you can't just think about that, you have to look at the positives. Think about what we were doing before.
"It will be difficult, yes, but it won't be impossible. Look what's happening between the first and second teams, only five points different. Did you expect that? If Atletico win then it's only two points. Did anyone expect that? The league still exists and I'm an optimist, always, until the very end."
Inconsistency has dogged Madrid this season, Zidane's side recording three heavy home wins in succession in La Liga, although they have failed to beat Fuenlabrada, Numancia and Leganes in the Copa del Rey at the Santiago Bernabeu.
And Zidane called for Madrid to renew their focus for Saturday's home game with Getafe, although attention is being drawn to Tuesday's clash with Paris Saint-Germain in the second leg of their last-16 Champions League tie.
"We've just lost a game and we want to do all we can to go and get a victory," Zidane said. "We've got to do the most we can on the pitch, we studied the opponent but we concentrate on ourselves. We have to go out and give everything.
"All the teams play well, it's the reality of playing against good, motivated teams. They'll be really focused, it's one of the biggest games of the year for them. We have to go into these games in the same way.
"Anyone can have an opinion and feelings about how the squad is, if we are in good shape or not. What I think is we have won nothing yet. What I like to think is the games we have coming up give us a chance to show our quality and prove ourselves.
"We played well in the first game [against PSG] and against Espanyol we played well too. But nothing is certain in football. You don't know what you can achieve and that will always be the same. Tomorrow is a big game and we have to show the form we're in."