MADRID: Ronaldo temporarily shrugged off his love-hate relationship with the fans to net his 100th goal for Real Madrid on Saturday but it wasn't enough as an Iker Casillas howler gifted Real Sociedad a point at the Bernabeu.
To make matters worse for Real Madrid, Valencia leapfrogged them into second place behind Barcelona in the table after beating Celta Vigo 1-0.
Valencia's win left them equal with Real on 59 points 10 behind Barca but in second on a better head-to-head record.
When Ronaldo netted with a sweet right-foot volley from a crossfield Zinedine Zidane pass on 25 minutes, former European champions Real looked bound for a win that would at least have added a modicum of pressure to runaway leaders and defending champions Barcelona, who are scheduled to visit Racing Santander later yesterday.
Instead, Casillas flapped at a Mikel Alonso lofted cross into the box and Mark Gonzalez on loan from Liverpool swept home the gift.
To compound Real's misery they played out the final four minutes with 10 men after Guti was sent off in a bad-tempered finale which had the home crowd booing their disapproval.
The one point was in contrast a blessing for relegation-threatened Sociedad as they drew level with fourth-bottom club Mallorca.
Ronaldo, who last week saved a point at old club Barcelona who can now win the championship next weekend if they beat Santander and then win at fellow Champions League semi-finalists Villarreal might have put the match beyond doubt long before the break.
He hit the post from close range and then ballooned the rebound over the crossbar off a pass from the left by Robinho.
At the other end Gonzalez had looked lively and drew a smart Casillas save on 11 minutes as Sociedad dug in hard.
The same player then headed wide and on the strike of half-time Casillas again found himself having to stop Gonzalez, this time an effort from distance, as the visitors showed Real that one goal was unlikely to prove enough.
Ronaldo was subbed after tweaking a leg muscle on the hour by Italian Antonio Cassano, who was on the mark in last month's derby success against Atletico Madrid.
Cassano strove to make his mark but was caught up in a sea of increasing mediocrity as the game began to lost its momentum after Casillas' dreadful mistake.
An exciting Robinho run 12 minutes from the end deserved greater reward than for a left-foot shot off target.
Guti then lost his head and referee Fernandez Borbalan, who also dished out six yellow cards, gave him a red for dissent.
In Galicia, Valencia took the lead against home side Celta Vigo after 30 minutes, Miguel Angulo latching onto a smart lay-off from Uruguay international midfielder Mario Regueiro to drive a snapshot under the body of goalkeeper Jose Manuel Pinto.
Pinto atoned for his first half mistake with excellent saves of a David Villa shot and a Regueiro follow-up six minutes after the restart, but Pinto's Celta team-mates could not break down the Valencia defence in a scrappy second-half and the game stayed 1-0.
Barca will travel to Santander with coach Frank Rijkaard resting Ronaldinho, who suffered a bruised leg against Benfica in midweek as Barca reached the Champions League semi-finals.