Just three minutes later from the equaliser, Casemiro finished after a Ronaldo flick at the near post from a set-piece.
Atletico Madrid, Valencia and Sevilla must all visit Real down the stretch too but it is an away fixture at Athletic that Zidane is rightly wary of given no team has won there since Ivan Rakitic grabbed Barca's victor seven months ago.
The former Castilla holding midfielder had also played a typical combative role in a tight battle at San Mames, standing up to Athletic's physically imposing Aduriz and Raul Garcia in particular.
"Casemiro has improved in a spectacular way", Marcelo said.
Marcelo told BeIn Sports after the game that his low-profile countryman had come through to become a really important player for the Bernabeu outfit.
Yeray Alvarez prodded a glorious chance wide following Garcia's knockdown but Madrid continued to look unsafe on the break, and Kepa was forced into a one-handed stop low to his left after Ronaldo was played in by Benzema.
Aritz Aduriz cancelled out a first-half Karim Benzema effort to leave the two teams level in the 65th minute, but Casemiro's close-range effort secured a vital victory for Los Blancos, who have moved five points clear of Barcelona on the same number of matches.
"Benzema and Ronaldo? I prefer to focus on how everyone played, although Karim had a great game".
"I think we dominated the play and we got close to Madrid's area, but maybe not with the precision we wanted".
The result ensures that Zinedine Zidane's team will be top of the table heading into the global break and leaves Barca needing a good result at Mestalla to keep the pressure on in the title race. "It is a hard place to go to but we will try to play well and win the game, doing as we've been doing up until now". "We are in the final stretch in La Liga".
"In the end, we looked for more balance with the 4-4-2". "But we had time to prepare for the match and it worked".
They knew they would be tested by an Athletic side who had the second-best home record in the league and had not lost at home since being beaten by Barcelona in August.
Questions have been asked of Benzema in recent weeks, with Alvaro Morata considered by many to be Real Madrid's best centre forward, but the French striker was at his best against Athletic Bilbao.
Dermot Corrigan is a Madrid-based football writer who covers La Liga and the Spain national team for ESPN FC.