Steve Magoffin struck twice before tea to keep Sussex firmly in the driving seat in their top of the table clash with Middlesex.
The Australian had bowled without much luck in the morning but returned to remove John Simpson and Neil Dexter and undermine a Middlesex revival.
The visitors had moved on from 76-4 to 169-5 when Magoffin trapped Simpson lbw for 16 and in his next over had Dexter caught at first slip by Ed Joyce for 34.
Middlesex also lost the obdurate Nick Compton for 30 in the afternoon session as they went to tea on 192-7.
Compton had faced 102 balls when he edged a delivery from Ollie Robinson and was superbly caught by Mike Yardy at second slip.
The only disappointment for Sussex was the sight of Ajmal Shahzad having to leave the field for treatment to a shoulder problem soon after lunch.
Sussex will be hoping Shahzad’s problem is not too serious with seamers Tymal Mills, Lewis Hatchett and James Anyon already sidelined through injury.
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