Tonali sinks 10-man Chelsea as Newcastle win top five showdown
Tonali put Newcastle ahead after just two minutes at St James' Park before Chelsea striker Nicolas Jackson was sent off for an elbow on Sven Botman late in the first half.
Bruno Guimaraes struck in stoppage-time to secure the Magpies' seventh win from their last nine league games and lift them into third place in the Premier League.
Eddie Howe's side, aiming to feature in the Champions League for the second time in three seasons, are one point above fourth-placed Manchester City and three clear of fifth-placed Chelsea and sixth-placed Aston Villa.
League Cup winners Newcastle travel to Arsenal next weekend, before hosting Everton in their last two matches.
"I thought first half we were really good. I was very pleased. The sending-off changed the game," Howe said.
"With the score line as it was, it was a tricky second half for us mentally because we wanted to go and attack. But Chelsea tweaked things to make it difficult for us.
"We hung in there. We got over the line and that's the major thing in a game like today."
Chelsea's first defeat in seven league games leaves their bid to return to Europe's elite club competition in jeopardy.
Seventh-placed Nottingham Forest will move one point above Chelsea and knock them out of the top five if they beat relegated Leicester at the City Ground later on Sunday.
With Manchester United at home and Forest away left on their schedule, Enzo Maresca's team face a tense finish to their top five challenge.
Reaching the UEFA Conference League final against Real Betis with a win against minnows Djurgarden on Thursday was little consolation to Chelsea after this damaging result.
"It's already difficult 11 v 11 in this stadium, against this team so playing an hour with 10 players is not easy," Maresca said.
"The only thing I can say is that sometimes it is too easy to take some decisions in some stadiums. But if the referee says it's a red, it's a red.
"We were in a good run but my message was always the same to the players. The game is gone, think about the next one."
Newcastle made the perfect start after just two minutes with a blistering break that caught Chelsea cold.
Countering from a Chelsea corner that came to nothing, Newcastle's Anthony Gordon was felled by Moises Caicedo.
When the penalty appeal came to nothing, Jacob Murphy whipped in a low cross that Tonali converted at the far post.
Moment of madness
Swept along by the tidal wave of emotion rolling down from the stands, Newcastle poured forward in search of a second goal.
Alexander Isak miscued with the goal at his mercy after Dan Burn headed down Tonali's free-kick.
Chelsea were in dire straits and Jackson saw red in the 36th minute as his moment of madness encapsulated their haphazard start.
The forward led with his elbow as he crashed into Newcastle defender Botman, initially earning a booking that was upgraded to a dismissal after VAR called for a check.
Maresca sent on Reece James for Noni Madueke at half-time in a bid to remedy Chelsea's defensive issues.
Chelsea finally forced Nick Pope to make a save when Marc Cucurella took Cole Palmer's pass and unleashed a powerful effort that the goalkeeper palmed away.
That near-miss was part of a period of sustained Chelsea possession despite their numerical disadvantage.
Picked out by Jadon Sancho, Enzo Fernandez's drive forced a superb save from Pope in the closing stages.
James should have rewarded Chelsea for their late barrage but he headed wastefully over from an unmarked position.
Howe's anxiety was palpable on the touchline and Guimaraes finally eased the nerves in the 90th minute.
Burn's pass found the Brazil midfielder, whose shot took a hefty deflection as it looped in over Chelsea goalkeeper Robert Sanchez.