Thomas Tuchel conceded his previously unbeaten Chelsea side lacked confidence against Manchester City, as they suffered a 1-0 defeat at Stamford Bridge.
Chelsea went into the fixture on the back of consecutive 3-0 wins in the Premier League and positive results in the Champions League and Carabao Cup, but they were pinned in their own defensive third for long periods of the first half and fell behind to a Gabriel Jesus strike on 53 minutes.
Tuchel's side went in search of an equaliser and the game opened up but they could not find a response.
Chelsea had won their previous three games against Man City, including last season's Champions League final, but afterwards Tuchel conceded his side's approach on the day was affected by the quality of their opposition.
"We were excellent in the last 20 metres of the pitch but not the other 80 metres," he said. "City were stronger, sharper, with more position.
"I had the feeling we lacked belief to escape in the situation. With every mistake we did we lost more confidence. We defended very well until the goal, although we were too deep. There was no connection [with Romelu Lukaku] and this was a team problem, not an individual problem.
"We could not escape the pressure. It was a question of decision making and we were so unprecise. We lacked confidence to switch sides. We were not on our highest level and you cannot expect a result."
Chelsea had established themselves as a team among the favourites for the Premier League title with their start to the season and had the opportunity on Saturday to open up a six-point gap on the defending champions.
Tuchel said that may have played on the minds of his players.
"We played with the mentality we had something to lose but there was nothing to lose," he said.
"After the goal there was a good reaction and a bit more risk. That's the mentality you need from the start: that you take risks and accept risks. But they made us underperform."
Guardiola: It was fantastic!
Meanwhile, Pep Guardiola was full of praise for the style of his team's victory, praising the manner of City's performance away to a title rival.
The Spaniard says the result gives his players the belief they can mount a successful defence of their Premier League crown.
"From the first second to the last second we tried to play our game and it was fantastic," he said. "We played really good. OK we've three more points and that's really good for us. Three more points gives us the awareness we can do it again. We are here and we will try to do it again."
The victory also took Guardiola to a Manchester City club record. It was his 221st win in charge, moving him above Les McDowall, who managed the team in the 1950s and 1960s and won the FA Cup in 1956.
"I'm proud," Guardiola said when asked about the record. "In these 221 games I never scored one goal. I share it with all the people in the club, the players, all the staff. I can't deny how proud we are to achieve that, to be close to Les McDowall, the previous manager who achieved this record, and we try to do it more."