Arsene Wenger claimed the Premier League's congested fixture schedule had an impact on Arsenal's failure to win at Sunderland.
The Gunners were held to a 0-0 draw at the Stadium of Light less than 72 hours after their 2-0 victory against West Brom on Thursday night.
"I think our first half was very good," Wenger said. "The second half is always more difficult when you play Sunday-Thursday-Sunday. We created less but could still have scored. We should have been in front in the first half."
The result leaves Arsenal in fourth position on 64 points - level with Manchester City but with an inferior goal difference.
Fifth-placed Manchester United have 59 points but also have a game in hand in the race for Champions League qualification.
Asked if he was getting worried, Wenger said: “Yes, of course we worry. It is a fight.”
The Frenchman hinted that Arsenal have a tougher run-in than their rivals, adding: “We play against good teams.
In pictures - Sunderland 0-0 Arsenal:
“There are two leagues at the moment. In one there are the teams who are taking it easy, in the other the teams who are going for something at the top and the teams that fight not to go down.”
Arsenal were mathematically ruled out of the title race by Leicester’s thrashing of Swansea later in the day, and Wenger conceded his disappointment: “It is frustrating because we play to win the title.”
The Gunners will aim to get back to winning ways at home to Norwich next weekend, and Sunderland boss Sam Allardyce backed them to stay in the top four.
He said: “They will qualify for Europe for the 19th year on the trot. I would just like to be fourth-bottom!”