Real Madrid's players celebrate after forward Karim Benzema (2R) scored their team's first goal during the Spanish league football match against Athletic Club Bilbao 18, 2017
Casemiro was Real Madrid's unlikely hero as he hit the winner to open up a five-point lead on Barcelona at the top of La Liga with a 2-1 win at Athletic Bilbao on Saturday.
Karim Benzema fired the visitors into a half-time lead from Cristiano Ronaldo's cross.
Aritz Aduriz briefly levelled for Athletic, but Ronaldo again was involved for the winner on 68 minutes as his flick-on was slotted home by the unmarked Casemiro for just his third goal of the season.
Barcelona can cut the gap back to two points when they face Valencia at the Camp Nou on Sunday.
Athletic were undefeated in 19 games at home in all competitions stretching back to August and started confidently with Inigo Lekue threatening early down the left.
His dangerous cross was bundled just wide by Inaki Williams inside the first two minutes.
However, Madrid settled after the first 10 minutes and had the ball in the net in their first serious attack only to be denied by the offside flag.
Dani Carvajal timed his run to meet Benzema's through ball but he squared to Ronaldo when going for goal would have been the better option, and the World Player of the Year's tap-in was ruled out.
Six minutes later Ronaldo was denied again, this time by a stunning save from Kepa Arrizabalaga as the young goalkeeper leapt low to his left to turn the ball to safety.
Sergio Ramos should have continued his remarkable goalscoring streak when he then headed wide when unmarked from a Toni Kroos free-kick.
Madrid finally made the breakthrough on 25 minutes as Casemiro's ball over the top for Ronaldo broke Athletic's offside trap and the Portuguese produced an inch-perfect low cross for Benzema to slot home his sixth goal in as many games against Athletic.
Real continued to threaten on the counter-attack as Arrizabalaga was called into action once more to save Gareth Bale's drive from the edge of the area.
However, the visitors had to withstand an aerial bombardment at the start of the second half.
A mix-up between Keylor Navas and Ramos nearly saw the latter turn a loose ball into his own net.
The pace of Williams down the right was a constant threat for Athletic and he finally got his reward 25 minutes from time when his cross was headed back across goal by Raul Garcia and nodded in from close range by Aduriz.
The lead lasted just three minutes, though, as for the umpteenth time this season, Madrid found a winner from a corner.
Kroos's delivery was flicked on by Ronaldo and midfielder Casemiro was completely alone inside the six-yard box to slot home from close range.
Marcelo should have put the game to bed 18 minutes from time when be blasted over from point-blank range after a brilliant team move involving Benzema and Ronaldo.
Ronaldo was surprisingly replaced by Isco 12 minutes from time, but Zinedine Zidane's decision was vindicated as Real held out to take a huge step towards a first La Liga title in five years.