Manchester United could be back at the Premier League summit by the time Ancelotti's men kick-off against Stoke next Sunday after Sir Alex Ferguson's men beat City in the early kick-off then saw Spurs win 2-1 against the Blues.
The results also mean Spurs are back in the Champions League places, with their victory over Arsenal in midweek backed up with goals from Jermain Defoe and Gareth Bale, while Chelsea had John Terry dismissed before pulling one back through Frank Lampard.
"We had a problem with soft defence," said Ancelotti.
"We didn't put pressure on the ball. Lampard scored at the end but when we lost Terry it was impossible to recover the game with 10 against 11."
Chelsea know they will win their first title since 2006 if they defeat Stoke, Liverpool and Wigan.
"We know what we have to do in the next three games," Ancelotti said.
"We must not have a panic now. We are top of the league with only three games left so every team wants to be in our place.
"We have two at home and one difficult game away. We can be the best again. A one-point lead is not bad at this stage of the season.
"I am always confident. I am less confident than yesterday but we are still in the best position."
Ancelotti made his final two substitutions at half-time before Didier Drogba pulled up with an apparent groin problem just before the restart.
"Didier never said anything at the end of the first half," the Italian added.
"He had a problem but after five minutes he was good to play to the end of the match without a problem. Now he doesn't have a problem."
Ancelotti also refused to comment on whether he thought Terry's dismissal, for fouls on Roman Pavlyuchenko and Bale, was harsh.
"We are disappointed because we have lost John Terry for the next game but I don't have to say anything about the decision," he said.
"Everyone is under pressure at this moment. He is the captain and involved in this exciting race."