Tuchel 'has no explanation' after Bayern Munich are humbled by Mainz
With Bayern in control and leading 1-0 at half-time after Sadio Mane's first goal since October, the visitors conceded three goals in 14 minutes.
The loss gave Borussia Dortmund a chance to go first with a win when they host Eintracht Frankfurt later on Saturday.
A "perplexed" Tuchel said his side "was missing the energy to fight back from setbacks".
"We look like a team that's played 80 games so far this season.
"We look worn out. We aren't in the condition to play without making mistakes.
"We're losing our order, we're losing our clarity. It's not clear enough over 90 minutes and we were ruthlessly punished.
"We're not in the position to fight back, to show the necessary energy to show a reaction.
"The points are falling through our hands like sand."
The loss places the focus back on the Bayern hierarchy's decision to fire Julian Nagelsmann (35) and replace him with Tuchel in late March.
Since taking over, Tuchel has overseen exits in the Champions League to Manchester City and the German Cup to Freiburg, with two wins from seven games in all competitions.
After the game, Bayern CEO Oliver Kahn (53) backed Tuchel, saying he "is the last person we need to discuss."
"He's doing everything with his coaching team, tactically and psychologically, to make the boys understand that it's worth fighting to win this German (league) title."
After the match, Tuchel did not share Kahn's assessment, saying he was at fault for the team's poor form.
"I'm always responsible. Regardless of what happens. We will analyse it together.
"We've lost an extreme amount of points in the second half of the season."
Tuchel said he would give his side three days off ahead of next Sunday's clash against rock-bottom Hertha Berlin.