Real approached the weekend ravaged by six injuries and smarting after their elimination in the King's Cup quarter-finals to Celta Vigo as their Spanish record-breaking 40-game unbeaten run was followed by one win in their last four matches.
Barca had won their last five games in all competitions but fell behind in the early kick off when Betis's Alex Alegria struck in the 75th minute and the champions needed a 90th minute leveller from Luis Suarez to snatch a point.
Sevilla, who ended Real's unbeaten run two weeks ago, had won five in a row in the league but got off to a nightmare start at Espanyol when defender Nicolas Pareja was dismissed in the second minute, foreshadowing a costly defeat.
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Real knew a win against Sociedad would give them extra breathing space and after a slow start Mateo Kovacic broke the deadlock shortly before the break, with goals from Cristiano Ronaldo and Alvaro Morata wrapping up the points.
"We needed a game like this, now we're playing very well and need to keep going like this," said Kovacic.
"We're four points clear and we have to take advantage of that and stay ahead until the end so we can win the league."
Real lead both Barca and Sevilla by four points and a game in hand, against Valencia in February, offers them a chance to further boost their bid for a first league title since 2012.
Real's Cup exit could also benefit their quest as they have seven days before their next game, at Celta on Sunday, while Barca face a semi-final first leg at Atletico Madrid on Wednesday before meeting Athletic Bilbao on Saturday.
"I'm very happy because we have a week to rest and work hard and think about the next game," said Zidane. "We'll have a couple of days off to rest and that'll be great for the players and then we have the week to work hard.
"But this (Sunday's win) is only one game and we have to keep going. We have to give everything and keep training well.
"It won't be easy and we're going to have difficult games, but today was very positive because of our performance and the three points," he added.
(Reporting by Richard Martin; editing by Ken Ferris)
