Madrid and Barcelona are locked in a frantic La Liga title race, level on points -- with Zidane's team top thanks to their head-to-head record -- with seven games left to play over just three weeks.
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"I won't coach for 20 years. I'll retire before that," Zidane said in a news conference ahead of Real Madrid's game with Espanyol on Sunday. "I'm an unusual coach, anything could happen. I'm not planning anything. It's the day-to-day that excites me. For how long, I don't know.
"In my head I've always been a player. I played for 18 or 19 years, and after that when people asked me if I'd be a coach, I said no. Then I decided to become one. This really wears you out. It won't be 20 years, that's for sure."
Madrid have been in good form this month, winning four out of four matches since La Liga's return, while rivals Barca have already dropped points in a 0-0 draw with Sevilla.
Part of that success has been an impressive defensive record, with just 21 goals conceded in La Liga this season, the fewest of any team.
"[Ramos and Varane] are the best pair around," Zidane said. "What they do is exceptional. I like to play football, but defence is the most important thing today. We have an advantage with the defence we have, and we have players who are committed. But defending isn't about four or five players. We've shown strength and attitude as a team."
Ramos has also contributed in attack, scoring eight league goals this season -- his best-ever tally -- including a free kick in Madrid's 2-0 win over Mallorca on Wednesday.
Ramos and Zidane played together in the 2005-06 season, which was Zidane's last as a player and Ramos' first at the club.
"He had quality and above all personality," Zidane said. "He wanted to win, he wanted to make history at this club. I'm not surprised at all by what Sergio has done. We're lucky to have a player like that. I hope he stays with us to the end."
Sunday's opponents Espanyol are bottom of the La Liga table, eight points from safety. They sacked manager Abelardo on Saturday, with sporting director Rufete now in temporary charge -- their fourth coach in a disastrous season.