Leicestershire 257 for 9 (Ackermann 63, Dearden 61, Rayner 4-58) trail Middlesex 349 by 92 runs
Ollie Rayner took four wickets in his first appearance of the season to keep Middlesex in control on the second day of their County Championship clash against Leicestershire at Lord's.
Earlier, Rayner had also pulled off a fine reaction catch at second slip to account for Foxes captain Paul Horton, with the visitors losing four wickets for 23 runs during the evening session.
Leicestershire closed on 257 for 9, still trailing by 92 runs after the Seaxes posted a total of 349 in the morning.
Resuming at 325 for 8, Middlesex had a fourth bonus batting point within their sights - but they missed out on that by the smallest possible margin.
Chris Wright had George Scott caught at second slip and then did enough to tip Tom Helm's straight drive onto the stumps, leaving non-striker Ethan Bamber short of his ground as the home side were bowled out.
Helm, who finished unbeaten on 37, then drew first blood when Leicestershire replied, trapping Ateeq Javid lbw in the seventh over before Horton and Hasan Azad added 50 for the second wicket.
James Harris broke that partnership, getting the final delivery of the morning session to move back and hit Azad's off stump.
Leicestershire seemed to be faltering at 80 for 3 soon after lunch, when Bamber was rewarded for his consistent line as Rayner snapped up a smart catch to dismiss Horton.
Mark Cosgrove survived a close shave when an inside edge zipped past his leg stump, but he then began to get into his stride. Having dispatched Bamber for two sweet straight drives to the boundary, he had reached 28 when he was pinned leg before by Rayner.
Dearden batted fearlessly - moving off the mark with a square cut for four and unleashing further savage shots on the off side as he soon caught up with the generally more pedestrian Ackermann. It was Ackermann who passed his half-century first, edging Bamber to third man for four and Dearden followed suit, registering his first County Championship 50 of the season later in the same over.
The left-hander had reached 61 from 106 balls when he drove at Rayner and perished to a stunning left-handed catch by Middlesex captain Stevie Eskinazi at short cover. Ackermann was next to depart, pushing forward to a flighted delivery from Rayner and getting an outside edge to John Simpson.
Rayner collected his fourth wicket with a low caught and bowled to dismiss Tom Taylor and Helm removed Ben Mike, caught behind for five, in the final over of the day.