The Portuguese winger netted the opener inside the opening minute with a searing run and shot and Xerez, in their first top-flight campaign, must have feared the worst.
But the floodgates remained stubbornly closed until the last 15 minutes when Ronaldo, Guti, Karim Benzema and Ruud van Nistelrooy netted in quick succession to settle a restless home crowd.
"We didn't play well as a team but I am pleased with the final result," Real coach Manuel Pellegrini told reporters. Real moved ahead of champions Barcelona and Athletic Bilbao on goal difference at the top of the standings with three wins from three.
Barca hammered Atletico Madrid 5-2 on Saturday and Bilbao were 3-2 winners over Villarreal earlier in the day.
Real Madrid appeared to ease off after Ronaldo's early strike at the Bernabeu and Xerez steadily grew in confidence.
Emilio Viqueira skimmed Iker Casillas's crossbar with a free kick and Xerez's travelling fans cheered 'ole' as the visitors were given time and space to move the ball around, though they lacked the quality to really threaten.
REAL SUBSTITUTES
Pellegrini brought midfielders Guti and Esteban Granero off his bench midway through the second half and they helped turn the game.
Ronaldo bagged the second, rising to head in from a corner in the 75th minute and Guti volleyed a third soon after.
Benzema lashed a spectacular fourth on a breakaway before substitute Van Nistelrooy slid the ball between Renan's legs for the fifth.
Spain striker Fernando Llorente nodded in Bilbao's opener and fired in a second off the post before the break at San Mames. He was only denied a hat-trick by acrobatic saves from Diego Lopez in the visitors' goal.
Cani pulled one back for Villarreal early in the second period and after Javi Martinez had restored the home side's advantage with a header from a corner, Santi Cazorla netted a fine free kick near the end.
David Villa had scored twice to set Valencia on course for their third straight win, after coming from behind to lead 2-1 at home against Sporting Gijon.
But the visitors, who had been down to ten men from the 25th minute following Michel's red card, snatched a late goal through Gregory Arnol, who headed in at a freekick after some poor goalkeeping from Miguel Angel Moya.
Elsewhere, Almeria beat Getafe 1-0 and Real Zaragoza slumped to a 2-1 home defeat by Real Valladolid.
