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 from Reece James’ corner into the lead 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.
Isak had failed to score in his five top-flight appearances since making a British record £125 million (B5.3 billion) 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.
Man Utd hit back
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.
“The pace and intensity was better in the second half. We controlled the game. We took advantage of a tired opponent.”
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 the 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 here 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 clean the air.”
Danger
On Saturday, Phil Foden spared Manchester City’s blushes after the Premier League title challengers blew a two-goal lead in a dramatic 3-2 victory over Leeds.
Pep Guardiola’s side were in danger of losing more ground in the title race following a second-half collapse at the Etihad Stadium.
Foden and Josko Gvardiol put City two up before the interval, but third-bottom Leeds hit back through Dominic Calvert-Lewin and Lukas Nmecha.
It was a much-needed win after consecutive defeats against Newcastle last weekend and Bayer Leverkusen on Tuesday.
“It was a relief,” Guardiola said. “The game was not perfect in the first half but it should be over with the chances we had.
“Football is emotions. We conceded the second and after we made a step-up. In the end it’s the quality of Phil.”
City had made their worst start to a campaign for five years and their second-worst under Guardiola.
But they took just 59 seconds to move in front as Foden fired a close-range strike in off the bar.
Leeds had no answer to City’s slick start and Gvardiol looked to have put the hosts in complete control with a tap-in from Nico O’Reilly’s header in the 25th minute.
Calvert-Lewin made an immediate impact after coming off the bench, reducing the deficit with a clinical finish after pouncing on Matheus Nunes’s woeful pass inside the City area in the 49th minute.
Leeds equalised in the 68th minute when Nmecha’s penalty - awarded for Gvardiol’s foul on Calvert-Lewin - was saved by Gianluigi Donnarumma before the forward recovered to score from the rebound.
In stoppage-time, Foden cleverly made room in the area for an ice-cool finish as Guardiola breathed a sigh of relief.
Spurs slide
At the Tottenham Hotspur Stadium, Thomas Frank’s men made a disastrous start against Fulham as Kenny Tete put Fulham ahead before Harry Wilson’s superb strike punished a calamitous blunder from goalkeeper Guglielmo Vicario.
Tottenham were booed off at half-time and although Mohammed Kudus pulled one back after the interval, there were more jeers at the final whistle.
With only three wins in their last 13 games in all competitions, Tottenham have slumped to 10th place.
Tottenham are without a home league victory since the opening weekend of the season, wining only four of their 10 matches in front of their fans in all competitions this season.
Frank was critical of the fans who taunted Vicario, saying: “I didn’t like that our fans booed at him. They can’t be true Tottenham fans because everyone supports each other when you’re on the pitch and we do everything we can to perform.
“I’m fine with them booing after the match, no problem, but not during. That is unacceptable in my opinion.”
Elsewhere, Newcastle celebrated boss Eddie Howe’s 48th birthday with a 4-1 rout of Everton at the Hill Dickinson Stadium.
A first away league win since Apr 7 was the ideal present for Howe, whose side made an explosive start when Malick Thiaw headed home from Lewis Miley’s corner after just 52 seconds.
It was the fastest goal in the Premier League this season.
Beaten in their previous four away games, Newcastle netted again in the 25th minute when Everton goalkeeper Jordan Pickford allowed Miley’s shot to squirm past his woeful attempted save.
Nick Woltemade made it three with a cheeky chip over Pickford in the 45th minute.
Thiaw’s 58th minute header added to Everton’s misery and Kiernan Dewsbury-Hall’s 70th minute eye-catching strike was little consolation for David Moyes’s men.
Brentford striker Igor Thiago reached 11 league goals this season as his double fuelled a 3-1 win against second-bottom Burnley in west London.
Sunderland climbed to fourth place after fighting back from two goals down to beat Bournemouth 3-2 at the Stadium of Light.