At 2.30pm Chelsea looked like they would be three points ahead of Manchester United when they kicked off against Tottenham, but then came Paul Scholes' last-gasp winner against City and a defeat for Ancelotti's men at White Hart Lane.
It means United could be back at the summit before Chelsea face Stoke in a match where they will be without skipper John Terry following his red card against Harry Redknapp's side.
Sir Alex Ferguson, who famously called this period of the season "squeaky bum time", has the momentum but Ancelotti said: "We are top of the list 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.
"To be sure we have to win all three games. Now the most important is against Stoke."
It was a wretched evening for Terry against Spurs, starting with a handball that led to a penalty and Jermain Defoe smashing home despite being stripped of spot-kick duties earlier in the season.
Gareth Bale doubled the lead, then Terry was involved in a verbal spat with a fan next to the tunnel as he walked towards the dressing room. The fan was ejected and banned for Spurs' next home game.
Terry will also miss Chelsea's next clash through suspension as he was dismissed in the second half for two bookings, although he said "twice, I won the ball" to his bench as he headed off.
Being without Terry as Stoke launch their long throw-ins into Chelsea's penalty area next weekend may increase the pressure on Ancelotti's defence.
The Italian, however, said: "It is normal to have pressure at the end of a Premier League season. We are at the top of the table and we have less pressure than the other teams.
The good news for Ancelotti was having Ashley Cole back on the bench following a serious ankle injury, while Michael Essien could also play some part in the run-in.
"Ashley Cole, yes, he is ready," Ancelotti said. "We hope Essien will be back before the end of the season.
"It's very exciting, I always said it will be decided in the last game, until then it will be a fight."