Borussia Dortmund head coach Lucien Favre said his team underestimated Mainz in their surprise Bundesliga loss.
Dortmund's grip on second position in the table loosened after suffering a shock 2-0 defeat against struggling Mainz on Wednesday.
After Bayern Munich secured an eighth consecutive league crown on Tuesday, Dortmund were below their best at home to Mainz midweek.
Dortmund succumbed to goals from Jonathan Burkardt and Jean-Philippe Mateta at Signal Iduna Park as Favre's men ending the day just three points ahead of RB Leipzig with two rounds remaining.
"I think we underestimated them," Favre said post-match. "So far we've always tried our hardest in every game and every game was tough. Very tough. Especially at the start of the season."
The Bundesliga resumed in May after the 2019-20 season was suspended in March due to the coronavirus pandemic.
"Of course the season has been long, now one and a half months longer than usual," Favre said. "Sometimes it's like back in school and you just count the days until it's summer. But we are not in school.
"Until today we always gave our best, today it was not good enough. We didn't defend as a team, which would have given you more courage in the offense, more possession of the ball. We couldn't do that.
"It is a great disappointment how we played today. We now have two more games left and can't through too much at the team now. That's too much. Way too much. And I know that. Let's focus only on the next match."
Next up for Dortmund is Saturday's blockbuster trip to third-placed Leipzig, who drew 2-2 at home to lowly Fortuna Dusseldorf.
Two goals in the space of three minutes from Kevin Kampl and Timo Werner put Leipzig 2-0 ahead in the 63rd minute, however, the hosts capitulated after conceding twice in the final three minutes.
Leipzig are three points clear of fourth-placed Bayer Leverkusen and four ahead of Borussia Monchengladbach in the race for Champions League qualification.
"This was not enough," Leipzig coach Julian Nagelsmann said. "Again we couldn't win at home. We could have secured out spot in the Champions League next season but failed.
"Now we have more pressure, even though we still have the best position. But it would have been nice to have it in the bag before we play a strong Dortmund side. Now we have to show out on Saturday."
Nagelsmann continued: "I haven't spoken to the team yet. I'll talk to them tomorrow [Thursday]. I don't think there is anything to gain by ripping them now. I know they didn't draw on purpose.
"We have our next game in two days against Dortmund which will be very important for us. We'll stop talking about Dusseldorf now and focus on Dortmund and get our Champions League spot which now only has been postponed by two days."