League One leaders Wigan caused an FA Cup upset by beating Premier League side West Ham, who had Arthur Masuaku sent off for spitting, to reach the fifth round.

Striker Will Grigg scored in each half for the 2013 winners.

The Northern Irishman's early towering header opened the scoring before he added a second-half penalty.

Masuaku was dismissed early in the second half for spitting at Wigan midfielder Nick Powell.

Powell had made minimal contact with Masuaku on the touchline after closing down the defender and referee Chris Kavanagh wasted no time in reaching for the red card.

The double took Grigg's tally to a competition-leading six goals in five FA Cup games this season and 14 in all competitions for the Latics.

Cheyenne Dunkley had a goal ruled out for Wigan shortly after Grigg's opener, with home goalkeeper Christian Walton comfortably recording a 10th clean sheet in his past 14 games as West Ham failed to muster a shot on target.

The Hammers also lost midfielder Pedro Obiang to a knee injury in the first half.

The Latics have lost just three league games all season and were unbeaten in 14 matches in all competitions dating back to mid-November prior to kick-off.

And you can see why they have let in just 13 goals in 27 League One matches this season, scoring 53.

The home side were at full strength and took the game to an already injury-hit West Ham side who made six changes from their previous Premier League outing.

Wigan were hungrier than West Ham and first to the ball. The home side enjoyed just over 56% possession and restricted West Ham to just two off-target shots.

Grigg was the star of the show with two goals for Paul Cook's side, although Powell deserves a mention.

Nathan Byrne provided the deep back-post cross for Grigg to power a close-range header past Joe Hart after just seven minutes.

Grigg again beat Hart from the penalty spot after Reece Burke was harshly adjudged to have blocked the ball with his arm just inside the penalty area.

Speaking on BBC Radio 5 live sports extra, former Southend manager Phil Brown hailed Wigan as being "streets ahead of West Ham".

West Ham can now truly fully concentrate on their Premier League survival. The six changes backed up manager David Moyes' pre-match comments that retaining their top-flight status was the main focus.

Key games with Crystal Palace, Brighton and Watford are on the horizon in the next three games, but the latest injury to Obiang and red card for Masuaku means Moyes could be without more first-team regulars.

Masuaku is certain to miss the coming fixtures, with Leeds United forward Samu Saiz banned for six games for spitting during their FA Cup third-round defeat by Newport.

It is understood Obiang did not need to go to hospital after the strong but fair challenge by Max Power towards the end of the first half, but his availability for Tuesday's visit of Palace remains to be seen.

The Hammers are unbeaten in their past five Premier League games, but their 11th place with 14 games remaining is far from secure with just four points between them and third-bottom Southampton.

England manager Gareth Southgate was in the stands, presumably to watch goalkeeper Hart. The Manchester City loanee was assured, and could do little about Grigg's opener from inside the six-yard area or the spot-kick.

Wigan striker Will Grigg on Final Score: "From start to finish we were the best side. We had lots of energy, played with a high tempo and thought our quality today was top draw.

"With confidence so high at the moment it was a great day for us.

"We don't mind who we get next. Obviously it would be nice to get one of the big boys but we'll play anyone."

Wigan resume their League One promotion bid at Rochdale on Tuesday (19:45 GMT), the same night West Ham entertain Crystal Palace in the Premier League ( also 19:45).