Chelsea midfielder Moises Caicedo was sent off in the 38th minute at Stamford Bridge for a crude foul on Merino that caught the midfielder on his ankle.
Despite Caicedo’s dismissal, Trevoh Chalobah headed Chelsea into the lead from Reece James’ corner in the 48th minute.
But Merino’s close-range header from Bukayo Saka’s cross in the 59th minute 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.
The Gunners 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.
“I think we showed we are heading in the right direction. We were a much better team than them 11 v 11 but with 10 players, it is difficult but we dealt with it outstandingly,” Chelsea boss Enzo Maresca said.
Arsenal manager Mikel Arteta added: “Two teams that play with a lot of intensity and aggression, you could sense it in every tackle.
“We scored a brilliant goal. We had two or three big chances. It’s been a massive week emotionally and physically.”
Isak had failed to score in his five top-flight appearances since making a British record £125 million move from Newcastle in September.
The Sweden forward banished that drought with only his second goal in 11 games in all competitions for the Reds, following his maiden strike in a League Cup win against second-tier Southampton.
Cody Gakpo bagged Liverpool’s second goal in stoppage time to secure their first win in four games in all competitions.
“It’s very important for us as a team that we went 1-0 up, but I think it was also important for him because I think it was his third or fourth chance of this game,” said Liverpool manager Arne Slot.
Joshua Zirkzee scored his first Premier League goal for nearly a year before Mason Mount grabbed the winner as Manchester United came from behind to beat Crystal Palace 2-1.
Ruben Amorim’s side trailed to Jean-Philippe Mateta’s first half penalty at Selhurst Park, but Zirkzee and Mount netted after the interval to secure United’s first win in four games.
United recovered from their lacklustre start thanks to Zirkzee’s first goal in the Premier League since he netted against Everton in December 2024.
“I could see that Palace were getting tired at the end of the first half and knew they would suffer if we scored,” Amorim said.
Unai Emery praised his Aston Villa players for turning their season around after a 1-0 win against bottom of the table Wolves moved them into fourth place.
Boubacar Kamara’s stunning strike secured Villa a seventh win from eight league games – a stark contrast to the start of the campaign when they failed to win their first five games and scored just one goal.
“Of course I was worried and I was responsible. I tried to share my worries with the players two months ago. The most important thing is how they responded to my message,” Emery said.
Fabian Hurzeler was happy to erase bad memories from the City Ground after Brighton won 2-0 at Nottingham Forest.
The Seagulls avenged the 7-0 defeat they suffered there back in February, with the victory helping them climb to fifth place.
Maxim De Cuyper and Stefanos Tzimas scored for Brighton and Hurzeler said: “We arrived here with some memories regarding last season but we came to clear the air.” (AFP)
