Arsenal boss Arsene Wenger admits conceding a late equaliser against 10-man Atletico Madrid left a "bitter taste" as his side passed up the chance to take control of their Europa League semi-final.
With the LaLiga outfit on the back-foot for much of the contest following Sime Vrsaljko's 10th-minute dismissal, the Gunners dominated the game and looked set to take a lead to Madrid next week when Alexandre Lacazette headed them in front.
The visitors, however, had other ideas and - despite being a man light and seeing manager Diego Simeone sent to the stands - Antoine Griezmann capitalised on weak defending to convert an 82nd-minute leveller and secure a 1-1 draw.
After announcing he will step down as Arsenal boss at the end of the campaign, this was Wenger's final European home game at the helm.
But he admits Arsenal now head to Spain in a "difficult position" after another profligate evening in front of goal.
"We produced a performance we wanted to produce, but the result is not in line with the performance," Wenger said.
"We can only look at ourselves on tonight's result because 1-0 is a perfect result at home. It was down to us after that not to make a mistake.
"We had 20 shots on goal, we have seen that against Manchester United (earlier in the season), where we had 33 shots on goal and lost the game. It's a story we've seen before.
"For us the task is clear. But of course you come out tonight with a bitter taste, because we had the chances to be in the final tonight.
"That's where the regret is. We were not clinical enough and unfortunately we gave a goal away which puts us in a very difficult position."
Arsenal now have to win in Madrid or draw the game at least 2-2 to advance to the final, with Wenger ready to set out to attack at the Wanda Metropolitano.
"The only advantage of that result is we know exactly what we have to do over there," he added.
"We have nothing to lose. We have to go there and really play to win the game."
Simeone watched much of the game from the directors' box after he fumed on the touchline in the early stages.
The Argentinian spoke after the match and expressed his excitement of what promises to be an intriguing second leg next week.
"It's tough to know what to say," he said. "The ref decided to send off Vrsaljko and I thought an Arsenal player should have got a booking. When I protested he sent me off.
"To be honest it was 90 minutes in which my body was full of strong emotions, these players are basically heroes.
"They stood firm, after a long season, for 80 minutes in a tremendous way. I have goosebumps thinking about the next match."