Chesterfield were the better side but Millwall the better finishers as they left the Proact with a 2-1 win.
Sam Morsy deservedly put Spireites ahead on 25 minutes to cap a real purple patch of play when the skipper hammered home his first goal since September from 30 yards.
But the home lead was soon nullified as Aiden O’Brien headed home from Shane Ferguson’s cross.
Lee Gregory then hit the winner two minutes after the break to send Spireites home empty-handed.
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