Ernesto Valverde's men were in danger of falling to their first league defeat of the season when Gerard Moreno nodded the hosts ahead midway through the second half, but centre- back Pique met a Lionel Messi corner in the 82nd minute to snatch a point.
Barcelona are now nine points clear at the top of the table from Atletico Madrid, who beat third-placed Valencia 1-0 at the Wanda Metropolitano.
"Even though it was raining, we thought that the pitch would hold, but then we saw that the grass was not going to hold up and that the pitch was becoming dangerous," the Barcelona coach said.
"On our passes from defence the ball could get stuck in a puddle and that's why things got difficult. We give a lot of value to this draw, because it was hard to get because of this scenario and because it's a derby and we didn't want to lose."
The five-time European champions also set a new club record of 22 matches unbeaten to start a league season on Sunday, and are still in the hunt for a La Liga, Copa del Rey and Champions League treble.
They were far from at their fluent best against a stubborn Espanyol whose point drags them eight clear of the relegation zone, although Philippe Coutinho came close to scoring his first goal since joining from Liverpool last month when he struck the crossbar in the first half.
Faltering champions Real Madrid are 19 points adrift of arch rivals Barca in fourth, after throwing away two points in a 2-2 draw at Levante on Saturday.
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Atletico took advantage of Barcelona's draw to revive their slim title hopes, as Angel Correa produced a moment of magic to win the game.
Neither side managed to create many chances in a sloppy first half, with Saul Niguez seeing a long-range effort palmed away by Valencia goalkeeper Neto, while Simone Zaza dragged a shot wide for the visitors.
But Correa stunned Neto with a magnificent long-range strike when nothing appeared on in the 59th minute, with the ball flying into the top corner from 25 yards out to give the 22-year-old Argentinian his sixth league goal of the season.
But star forward Antoine Griezmann was whistled by the home fans late on, when he opted to run down the clock instead of pushing for a second goal.
The Frenchman has had an up-and-down relationship with the Atletico supporters since saying last year he wanted to leave the club in the future.
"Griezmann did what he had to do... That's normal, people want to win," coach Simeone said.
"The players are the ones who best interpret what they have to do on the pitch. Griezmann made only good choices tonight, even though he didn't score and couldn't finish.
Earlier on Sunday, Girona moved into the top half of the table as Uruguayan international Cristhian Stuani scored a double in a 2-0 win over Athletic Bilbao, while Getafe and Leganes played out a goalless draw.
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