Chelsea midfielder Moises Caicedo was sent off for a crude foul on Merino late in the first half at Stamford Bridge.
Despite Caicedo’s dismissal, Trevoh Chalobah headed Chelsea into a second-half lead, but Merino’s leveller ensured Arsenal emerged unscathed from a bruising encounter between the title rivals.
Arsenal are five points clear of second-placed Manchester City, who beat Newcastle on Saturday, and sit six points above third-placed Chelsea.
When the dust had settled on a contest of relentless intensity, Arsenal were left to rue a missed opportunity to extend their lead over Chelsea in the title race, while the Blues were relieved to avoid a damaging defeat.
Arsenal remain the favourites to win their first Premier League crown since 2004, but Chelsea’s combative display suggested they could emerge as the biggest threat to the Gunners’ title aspirations.
Arsenal are unbeaten in 17 games in all competitions, winning 14 of those matches, while Chelsea have gone seven matches without losing in all competitions.
Chelsea swept to a 3-0 win over Barcelona in the Champions League on Tuesday, while Arsenal enjoyed a 3-1 victory over Bayern Munich 24 hours later.
The positivity from those results was quickly forgotten as a bitter battle broke out at the Bridge.
Chelsea boss Enzo Maresca had played down his young side’s title hopes, but they went toe to toe with Arsenal, who were without injured centre-back William Saliba.
A thunderous first half included a rash of bookings as both teams tried to impose themselves.
Gunners midfielder Martin Zubimendi hauled down Reece James, Chelsea left-back Marc Cucurella crunched into Bukayo Saka, Mosquera pole-axed Joao Pedro, and Riccardo Calafiori cynically tugged James.
Saka almost exacted immediate revenge on Cucurella with a stinging strike that Robert Sanchez saved at his near post.
Teenage sensation Estevao Willian started for Chelsea after his star role in their win over Barcelona.
The 18-year-old couldn’t replicate his stunning goal against Barca however, as he lashed over from 10 yards to squander Chelsea’s first serious chance.
Enzo Fernandez tested Arsenal keeper David Raya from the edge of the area as Chelsea began to exert pressure on the visitors’ reshuffled defence.
The war of attrition turned ugly in the 38th minute when Caicedo caught Merino on the ankle with a nasty foul that was upgraded from a booking to a dismissal after a VAR review.
It was the Blues’ sixth red card in all competitions this season.
The Ecuador midfielder’s premature exit was followed by a dangerous Hincapie elbow on Chalobah, provoking Chelsea cries for a red card that went unheeded.
Gabriel Martinelli nearly added to Chelsea’s angst on the stroke of half-time with a fierce blast that forced a fine save from Sanchez.
Arsenal arrived as the best set-piece team in the league with 10 goals in 12 matches.
But Chelsea ranked second with eight and Chalobah grabbed their ninth in the 48th minute.
It was a goal straight from the Arsenal playbook as James curled a corner to the near post and Chalobah rose highest inside the six-yard box to glance his header into the far corner.
Arteta responded by sending on Martin Odegaard and Noni Madueke, who was jeered and barraged with chants of “Chelsea reject” on his return to his former club.
The Arsenal changes reaped an instant reward as Merino snatched the 59th minute equaliser.
Saka danced past Cucurella with a mesmerising run and cross, picking out Merino for a powerful close-range header that flashed past Sanchez.
It was Merino’s fourth goal this season as Arteta once again used the Spain midfielder as a makeshift striker.
A frantic finale featured Sanchez making superb stops to deny Saka and Merino, but Arsenal couldn’t land the knockout blow.
