Gabriel Martinelli scored his first hat-trick for Arsenal as the Premier League leaders beat second-tier strugglers Portsmouth 4-1 to reach the FA Cup fourth round on Sunday.

The Brazilian was criticised in midweek for his behaviour after Liverpool’s Conor Bradley suffered a serious injury near the end of their Premier League clash on Thursday but ended the week in style with his first goals in the FA Cup.

Manchester United bowed out though as they lost 2-1 at home to Brighton and Hove Albion with Danny Welbeck on target against his former club.

It is the first time since the 1981-82 season that United have failed to win a round of either domestic cup competition.

In other third-round ties, West Ham United enjoyed a break from their Premier League woes as they beat second-tier Queens Park Rangers 2-1 after extra time with new signing Valentin Castellanos heading the winner.

Premier League Leeds United beat Derby County 3-1 while third-tier Mansfield Town beat Championship side Sheffield United 4-3 away.

After several shocks already in the third round, including sixth-tier Macclesfield knocking out holders Crystal Palace, another one looked possible when Colby Bishop raised the roof at Fratton Park by putting Portsmouth ahead after three minutes against Arsenal, firing in a rebound after a save by Kepa Arrizabalaga.

“We started the game exactly how we didn’t want to, giving them hope and momentum but we managed to turn things around,” Arsenal manager Mikel Arteta, who made 10 changes to his starting lineup, told TNT Sports.

“We had to fight back and credit to (Portsmouth) because they make it difficult for us, especially in the early stages.”

ARSENAL ENJOY MORE SET-PIECE SUCCESS

Record 14-time Cup winners Arsenal were only behind for five minutes as Portsmouth were unable to deal with a set-piece delivery and the ball went in off home player Andre Dozzell.

Martinelli glanced in Noni Madueke’s corner in the 25th minute to put Arsenal in front and then hit the post soon afterwards from close range.

Madueke put a penalty wide just before halftime to keep Portsmouth in contention, but Arsenal eased away after the break as Martinelli converted from a superb pass by Gabriel Jesus.

He completed his hat-trick with a header as Portsmouth were again unable to defend from an Arsenal set-piece — the 17th goal Arsenal have scored from a corner this season.

Arsenal are still challenging for silverware on four fronts and face Chelsea in their League Cup semi-final first leg on Wednesday.

In contrast, Manchester United’s only focus now will be on the Premier League as they suffered more disappointment in interim manager Darren Fletcher’s second game in charge, and first at Old Trafford.

Having lost to fourth-tier Grimsby Town in their first match in the League Cup this season, the FA Cup represented United’s only remaining route to silverware. But Brighton deservedly claimed their first win over United in the competition.

Brajan Gruda’s 12th-minute goal gave Brighton the lead and Welbeck doubled their advantage with a thumping finish midway through the second half.

Benjamin Sesko gave United a lifeline but they could not find an equaliser and their misery was complete as youngster Shea Lacey was shown a red card for a second booking.

It is the first time 13-time FA Cup winners United have lost in the third round since 2014.