Here's a replay of Deniz Undav's early finish. Grimsby goalkeeper Max Crocombe won't want to watch this one back.

A beach ball comes on to the pitch, the referee tries to pop it with his studs but nearly falls over.

Brighton's corner is then easily cleared.

And there's the chance for Brighton as Kaoru Mitoma goes clear down the left. His cross is just cleared behind for a corner before Solly March can tap home.

Was Pascal Gross also pulled down in the area as the ball came in? VAR checks it but says no.

Leon Osman
Former Everton midfielder on BBC One

Grimsby have to try and be as perfect as they can.

Any mistake and Brighton are the kind of team that can punish you.

A rare foray upfield for Grimsby is ended with Michee Efete needlessly straying offside. It's not been a classic since the early goal.

Pressure dealt with for Brighton as Lewis Dunk heads the throw away before Evan Ferguson wins a free-kick.

Grimsby have a corner? No, throw-in right by the flag.

The visiting fans are making plenty of noise as Niall Maher prepares to launch one into the box.

Leon Osman
Former Everton midfielder on BBC One

Conceding early might not be the worst thing for Grimsby.

Brighton may just take their foot off the gas, thinking it is all comfortable. Stay in the game, build yourselves into it.

Grimsby are linking up a bit better, getting some touches as they begin to settle.

We're yet to have a shot since Brighton took the lead.

Grimsby looking disciplined, playing a very regimented 5-4-1.

Oh hello! Out of nowhere the Mariners are nearly gifted a goal as Lewis Dunk's back pass loops over Robert Sanchez's head but the Brighton keeper is able to sprint back before the ball reaches his six yard box.

Nervy.

Leon Osman
Former Everton midfielder on BBC One

It is important Grimsby don't concede again in the next 10 minutes to give themselves an opportunity.

It's all gone a little flat. Brighton keep the ball so well and they're happy to pass it around midfield, waiting for the next big chance.

Nearly a second for Brighton and Undav but a ball over the top is just beyond the German forward.

Grimsby have to be careful here, the game could slip away from them.

Leon Osman
Former Everton midfielder on BBC One

Simple from Undav.

He was the only man on the move. Grimsby's back three are flatfooted and he is exactly where you'd want your striker to be.

Back to the action and it's a proper midfield battle. Grimsby have had a couple of touches on the ball, going long but haven't managed to get in behind just yet.

A year ago Grimsby were a non-league club. Now they're in an FA Cup quarter-final.

A very handsome live-texter covered their National League play-off final win last June.

On that day Solihull Moors were the better team. What might have been...

Early chance for Brighton in the penalty area as teenager Evan Ferguson gets a yard of space before firing a harmless shot straight to Grimsby keeper Max Crocombe.

Peeeeeep. And we're off.

Grimsby are hoping to become the first ever fourth-tier team to reach an FA Cup semi-final. Both sets of players have just walked out to a wall of noise.