Middlesex in command in crucial promotion battle with Glamorgan
However, Middlesex’s momentum was punctured by a series of rain stoppages throughout the day and they eventually reached 286 for five, holding a handy lead of 72, when play was abandoned.
Simpson remained undefeated on 72 at stumps, with Ryan Higgins 35 not out at the other end.
Having resumed 82 in arrears, Middlesex progressed steadily during a truncated morning session, although Timm van der Gugten and James Harris both beat the bat on several occasions.
Simpson timed the ball sweetly, punching Van der Gugten through the covers to the short boundary and collecting four more from a straight drive off Harris as he and Stoneman ate into the deficit with some aggressive running between the wickets.
Stoneman was just four short of his hundred when play was halted by the first downpour of the day, leaving him to stew over an extended lunch interval.
But the opener lost no further time after the resumption, advancing to three figures from 179 balls and stepping up a gear as he dispatched Harris for a trio of boundaries in one over.
With the batting side swiftly moving into the lead, Glamorgan turned to spinner Ajaz Patel and, although Stoneman crashed his first delivery to the boundary, an uncertain prod at the second was gobbled up by Chris Cooke behind the stumps.
However, Simpson progressed to his half-century for the seventh time this summer and he and Higgins secured Middlesex’s second batting bonus point prior to two further rain delays – the second of which brought play to a premature end.
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