Leaders Barcelona followed up Saturday's 2-0 victory over Real with a 3-0 home win against Deportivo La Coruna on Wednesday to go six points clear on 83 but Real's success in Andalucia has trimmed the gap back to three with six games left.
Almeria came under some early pressure at their Juegos Mediterraneos stadium before an unmarked Albert Crusat gave the home side a surprise lead in the 14th minute, sweeping a low Kalu Uche cross past Real goalkeeper Iker Casillas.
"We still have a chance to win the league but we have to win all our remaining matches," the Netherlands midfielder, who will be a key member of the Dutch team at the World Cup, said in a pitch-side television interview.
"I had a lot of chances and missed two or three in the first half so I'm very happy with the goal," he added.
Real are out of the Champions League and the King's Cup and remain in danger of ending the season without any silverware despite an investment of close to a quarter of a billion euros in players for this season.
President Florentino Perez spent a record 94 million euros to buy Ronaldo from Manchester United and the 25-year-old's goal against Almeria was his 19th in the league this season, eight short of Barca's La Liga top scorer Lionel Messi on 27.
"At the start we were a bit flat because of what happened on Saturday (against Barcelona) but then we got into the match very well and created a lot of chances," Ronaldo told reporters.
"We didn't play a fantastic match but we managed to win and that's the important thing."
Real host third-placed Valencia on Sunday, with Unai Emery's side a massive 21 points behind them after their 2-0 home win over Athletic Bilbao in Thursday's late kick-off.
