Ronaldo scored his 100th goal but Real Madrid's 1-1 draw with lowly Real Sociedad on Saturday virtually snuffed out its hopes of catching Spanish league leader FC Barcelona.
Ronaldo, playing his 164th game in all competitions since joining Real from Inter Milan in 2002, volleyed in Zinedine Zidane's low free kick in the 26th minute at Santiago Bernabeu Stadium.
Sociedad equalized through Mark Gonzalez in the 62nd to leave Madrid 10 points adrift of Barcelona.
Barcelona now needs only nine points from its remaining seven games to ensure it retains its title.
Madrid, which last won a major trophy in 2003, was replaced in second position by Valencia, which edged Celta Vigo 1-0 on the back of Miguel Angel Angulo's goal.
Midfielder Jose Maria "Guti'' Gutierrez added to Madrid's difficulties in the 86th minute by insulting the referee and receiving a red card.
"We didn't expect this game. We had been making good progress in terms of our play, discipline and attitude,'' Madrid coach Juan Ramon Lopez Caro was quoted as saying.
"The team tried hard to reach our opponent's goal and win the match but we failed.''
Following discussions with Madrid president Fernando Martin after the game, Lopez Caro canceled an arrangement for the players to have two and half days off this week.
Barcelona leading
FC Barcelona leads the league with 69 points, followed by Valencia and Madrid with 59. Osasuna has 55 points, four more than Sevilla and Celta.
Madrid, without injured David Beckham and suspended Roberto Carlos, struggled as a lively Gonzalez fired several shots before Ronaldo scored his fourth goal in as many games.
The Brazilian limped off the field to applause in the 57th minute.