Crystal Palace boss Alan Pardew felt a slow start and lack of urgency paved the way for a 1-0 loss to West Ham in Saturday's London derby.
Manuel Lanzini scored what proved to be the winner after 18 minutes at a rain-lashed Selhurst Park, neatly converting a fine cross from the overlapping Aaron Cresswell.
Palace wasted a great opportunity to level just before the break, however, when Christian Benteke fired high and wide from the penalty spot having been brought down in the box by Angelo Ogbonna.
Benteke then struck the post with a header and Wilfried Zaha and substitute Connor Wickham also went close in the second half, but Palace were ultimately unable to find a way through even after West Ham lost Aaron Cresswell to a second yellow card after 75 minutes.
And although the defeat was just the hosts' second in eight games in all competitions and they remain eighth in the Premier League table, Pardew could not hide his disappointment.
"I'm angry and frustrated with the result," he said.
"It's a derby and the first 20 minutes they were brighter, faster, more aggressive. We talked about wanting a fast start, but we gave them a little gee up with that goal. Maybe we felt it would go well and when you're winning, maybe that happens.
"We bossed it after half-time, though, but West Ham defended admirably.
"But the bottom line is you have to be up for the task and we were waiting for it to happen. You can't do that in any sport, you've got to make it happen, so it's frustrating, because it's the start that cost us."
Pardew also singled out Benteke's penalty miss as a turning point in the game, with the former Liverpool striker failing miserably to add to the hat-trick he scored while on international duty for Belgium against Gibraltar earlier in the week.
"If we'd have scored the penalty, I think this is a game we win," he said.
"The penalty was a crucial moment. I'm expecting him [Benteke] to score after the week he's had. He puts the goalie on the floor, he has to hit the target."
Next on the agenda for Palace is next Saturday's away trip to reigning champions Leicester City.