Just 24 hours after leaders Bournemouth had fallen to a surprise 1-0 loss to struggling Leeds United, Ipswich - who started the day in third, just a point off the summit - were undone by a first-half blitz.
Sam Baldock fired Brighton ahead after 18 minutes and although Daryl Murphy levelled four minutes later, a Joao Teixeira double put the hosts in command before half-time.
Ipswich were well below their best, and despite a second from substitute Freddie Sears with 12 minutes left, they fell to only their fifth league defeat of the season.
There were also five goals at Craven Cottage, as Fulham striker Ross McCormack netted a first-half hat-trick to ramp up the pressure on under-fire Nottingham Forest boss Stuart Pearce.
Pearce - a Forest legend as a player - has come in for heavy criticism after a run that has seen the club slip down the table and his future will once again be under scrutiny following this 3-2 defeat.
McCormack was single-handedly responsible for sending Forest to a fifth loss in their last six games in all competitions, with his treble coming inside the first 35 minutes.
Forest did improve after Henri Lansbury had pulled one back on the stroke of half-time, but despite the former Arsenal man scoring again just past the hour, they did not have enough to snatch a late draw.
