Sussex 298 for 9 (Hamilton-Brown 77, Thomas 4-67) lead Somerset 76 (Magoffin 8-20) by 222 runs
Somerset were left nursing a major headache after another batting collapse saw them staring at a third defeat of the season after just the first day at Horsham. Having dominated champions Warwickshire at Taunton at the end of April, they now resemble relegation candidates rather than title challengers.
Director of cricket Dave Nosworthy admitted Somerset have an issue after his side were routed for just 76 by Steve Magoffin's career-best 8 for 20. They lasted only 23.4 overs as Magoffin ran through them, including a blast of five wickets in 13 balls.
"There are no excuses we didn't get it right," Nosworthy told somersetcountycc.co.uk. "We won the toss and chose to bat but it didn't work for us.
"I think the lads are all very disappointed, you train as hard as you do and try your best but at the end of the day you have got to be responsible for your own innings and get out there and perform, which nobody has managed to do for us today."
Nor have many managed to perform at all this season. A poor show with the bat in the opening match at Durham seemed to be an aberration for Somerset as they found form at The Oval and then racked up over 400 against a much-vaunted Warwickshire attack. But a slip at Headingley, where a draw was declared with Somerset 61 for 6 in their second innings, became a harsh reality after a thumping defeat at home to Middlesex.
Their batting order is ageing, inexperienced or out of form. They have been reliant on 37-year-old Marcus Trescothick and overseas player Alviro Petersen. Here, Petersen failed and Trescothick's 20 was one of only three double-figure scores.
Somerset can also not turn to their bowling attack for much comfort. The first three into the attack here, Peter Trego, Steve Kirby and Alfonso Thomas, have a combined age of 102. Together with Jamie Overton and Jack Leach - two youngsters whose development is essential for the club - they shipped 298 runs at over four an over to close day one 222 behind.
They had envisaged progress at a similar rate themselves with the bat but had no answer for Magoffin who bowled 11 overs unchanged to produce the best performance by a Sussex bowler since Mushtaq Ahmed took 9 for 48 when the county clinched the Championship title against Nottinghamshire at Trent Bridge in 2006.
He started by bowling Arul Suppiah through the gate - Suppiah's sixth single figure score in eight innings this season - then a touch of inswing proved too much for Petersen. But the real damage was done after James Hildreth was caught low down at slip in the 12th over.
From 42 for 3, Somerset subsided to 43 for 7, with Magoffin taking three wickets in four balls in the 14th over. Trescothick was caught behind playing away from his body before Trego and Thomas fell in successive deliveries.
Somerset's last three wickets put on 33 to avoid total ignominy but there was no stopping Magoffin. After Jack Leach was caught behind, Magoffin bowled top scorer Jos Buttler for 22 to pick up his eighth wicket and record the best bowling performance at Horsham since Northamptonshire's Vince Broderick took 9 for 35 in 1948.
Chris Jordan took the other two wickets and Sussex openers Chris Nash and Luke Wells were soon putting conditions into perspective with a stand of 83.
Thomas dragged Somerset back into contention when Nash, captaining Sussex on his home ground in the absence of Ed Joyce, played on before Joe Gatting edged his second ball to second slip.
A typically fiery burst from Kirby was rewarded with the wicket of Wells while Mike Yardy, who hit three successive balls from Kirby to the boundary, squandered a good start when he was bowled by Peter Trego at the start of a new spell.
But Sussex regained control in a fifth wicket stand of 85 between Rory Hamilton-Brown and Matt Machan before the latter was run out by Leach's direct hit from midwicket.
Somerset fought back after tea by taking five wickets, including Hamilton-Brown - whose 77 included 11 fours and two sixes and was his highest score since returning to the county before he became one of four victims for Thomas.
Despite the clatter of wickets, ECB pitch inspector David Capel said he was perfectly happy with the surface.