Arsenal overcame David Luiz’s dismissal against his old club to twice come from behind against Chelsea in an eventful London derby.
The Gunners looked beaten when Jorginho fired Chelsea ahead from the penalty spot after Luiz was shown a straight red card for bundling over Tammy Abraham.
The Chelsea forward had pounced on a terrible backpass by Shkodran Mustafi and rounded keeper Bernd Leno when he was shoved over by Brazil defender Luiz.
Chelsea were dominant for long periods but failed to kill the game off and were punished when N’Golo Kante slipped inside the centre circle allowing 18-year-old Martinelli to gallop free and produce an excellent finish, the Brazilian’s 10th league and cup goal of the season.
It was Arsenal’s first shot on target and came after a Chelsea corner which was cleared by Mustafi, the German atoning for his earlier error.
Chelsea thought they had won it when skipper Cesar Azpilicueta scored from six yards six minutes from time but spirited Arsenal earned a point through captain Hector Bellerin – making his first appearance since 5 December.
Resolute Gunners show fight
Arsenal have won just one of their last eight Premier League games and face the prospect of finishing outside the top six for the first time since 1994-95.
Mikel Arteta remains without an away win since taking charge in December yet they played better with 10 men and their gutsy second-half performance was sprinkled with positives.
Luiz, who has given away three penalties in the Premier League this season – more than any other player – was left with little choice after Mustafi’s under-hit pass let in Abraham who looked certain to score after going around Leno.
Yet Mustafi made up for his error with the clearance which allowed Martinelli to burst forward, after Kante’s slip, and produce a clinical finish under Kepa Arrizabalaga.
Chelsea looked like they had secured a first top-flight double over their London neighbours in four years when Azpilicueta restored their lead in the closing minutes but Arteta’s side were not done.
Having dropped 13 points from winning positions in the Premier League this season, they showed a determination that has been missing too often as Bellerin rifled in an equaliser to seal Arsenal’s fourth successive away league draw.
Wasteful Chelsea pay the price
This result will feel like a defeat for Frank Lampard and his Chelsea players.
Even before Luiz’s dismissal and Jorginho’s goal from the spot, the Blues dominated and should have been ahead after a hesitant start by the Gunners.
Having lost to a 93rd-minute goal at Newcastle on Saturday, Chelsea came flying out of the blocks.
Andreas Christensen headed wide from a corner when the Denmark defender should have done better while Abraham was six yards out when he headed another chance straight at Leno.
Callum Hudson-Odoi hit the top of the bar with a looping cross before Chelsea – who forced 17 corners and peppered Arsenal’s goal with 19 shots – eventually took the lead.
Yet Chelsea have managed just three clean sheets in the last 15 league and cup games and their defensive problems resurfaced as they failed to win for the seventh time in the league at Stamford Bridge this season.
On a disappointing night for the hosts, Abraham had to be helped off the pitch at full-time after colliding with an advertising board.
Both teams are in FA Cup fourth round action with Chelsea travelling to Championship Hull City on Saturday (17:30 GMT), while Arsenal are at Bournemouth on Monday (20:00).
More to follow.