LONDON, Jan. 11 (Xinhua) -- Premier League leader Arsenal overcame conceding an early goal to reach the fourth round of the FA Cup with a 4-1 win away to Championship side Portsmouth.

Gabriel Martinelli scored a hat-trick for Arsenal, who showed how effective they are from set pieces after Colby Bishop rewarded Portsmouth's bright start with a third-minute opener.

Arsenal equalized following a scramble after a corner in the seventh minute, and Martinelli scored his first after another corner in the 25th minute.

Although Noni Madueke missed a first-half penalty for Arsenal, Martinelli completed his hat-trick with a far-post finish following a well-worked team move and a header from another corner to keep the Gunners in the race for four titles this season.

Manchester United's bad season got worse after a 2-1 defeat at home to Brighton ended its interest in this year's FA Cup.

With Darren Fletcher still in temporary charge of Manchester United, his side was outplayed by Brighton, who led 2-0 thanks to goals from Brajan Gruda and Danny Welbeck before Benjamin Sesko gave the home side late hope with a goal in the 85th minute, although hopes of an equalizer were ended when Shea Lacey saw an 89th-minute red card.

Ben Brereton gave Championship side Derby Country a first-half lead at home to Leeds United, but the visitors turned things around with a quickfire double from Wilfried Gnonto and Ao Tanaka before James Justin confirmed the win with Leeds' third in injury time.

West Ham needed extra time for a much-needed win at home to second-tier Queens Park Rangers, with Crysencio Summerville putting the Hammers ahead in first-half injury time after Konstantinos Mavropanos suffered a serious injury.

Richard Kone levelled for QPR 20 minutes into the second half and the game went into extra-time where Taty Castellanos scored his first goal for West Ham to win the game for Nuno Espirito Santo's Premier League strugglers.

Elsewhere, Hull City won an all-Championship duel against Blackburn Rovers after a penalty shoot-out that followed 120 goalless minutes, Norwich City won 5-1 against fourth-tier Walsall, while Mansfield Town from League One caused a minor upset with a 4-3 victory away to Championship strugglers Sheffield United. ■