Zinedine Zidane's fears that Real Madrid's lack of intensity at the start of games would eventually cost his team have proved well founded.
On Wednesday night, away against a Valencia side that has endured its worst season in many years, Real showed a defensive frailty that allowed their opponents to score twice in the opening nine minutes on their way to a 2-1 upset of the La Liga leaders.
"That was our fault, " Real vice-captain Marcelo told the club website. "We started very poorly and there can be no excuses for it.
"We did all we could to at least have got a draw, but we conceded two daft goals and we must learn from our mistakes."
Just when it seemed Real might establish a stranglehold on the leadership, they find themselves still only one point ahead of defending champions Barcelona and now with just one game in hand.
"We still remain top but we have to change things, we can't be starting games like that," Marcelo said.
Worrying for coach Zidane is that his side, which had set a Spanish record 40-game unbeaten run in all competitions before falling 2-1 at Sevilla on Jan. 15, has now lost twice in six league games. Their last five away games have produced just one win in La Liga.
It was a missed chance to pile more pressure on a Barca team that has been on the ropes in the last week following a 4-0 humiliation at Paris Saint Germain in the Champions League.
Barca coach Luis Enrique and some of the players were booed on Sunday when they needed a 90th-minute Lionel Messi penalty to win 2-1 at home over strugglers Leganes.
"It hurts because it was a key opportunity to have extended our advantage at the top," Real captain Sergio Ramos told the club website. "But we've got to move on and think about our next game.
"La Liga season is a very long one."
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