Alexei Kervezee hit an undefeated half century for Worcestershire Seconds after Middlesex batsman Ryan Higgins had dominated the third day of the clash at Kidderminster with a triple hundred.
Kervezee ended undefeated on 65 from 121 balls with 11 fours and has so far featured in an unbroken stand of 50 with Zen Malik (23 not out) as Worcestershire closed on 143-3.
But the fourth wicket pair and the remaining batsmen will have to bat through the final day in a bid to stave off defeat after Higgins' marathon effort.
He began the day on 144 and was undefeated on 303 from 293 deliveries with six sixes and 36 fours when the declaration came at 670-5 declared – a lead of 373.
Higgins was aided in a fifth wicket stand of 260 in 50 overs by Tom Lake after Middlesex had resumed on 357-4.
Lake was eventually bowled by spinner Ben Twohig for 112 from 165 balls with 18 fours and then Don Manuwelge (67 not out from 75 deliveries) helped Higgins add an unbroken 129 before the declaration.
Worcestershire's first innings century-maker, Tom Kohler-Cadmore (9) was trapped lbw by a ball which kept low.
Ollie Westbury (34) partnered Kervezee in a stand of 69 before he was bowled and skipper Alex Hepburn (6) fell cheaply.
MIDDLESEX SECONDS – FIRST INNINGS CONTINUED
(overnight 357-4 off 98 overs)
Higgins not out 303
Lace b Twohig 112
Manuwelge not out 66
Total (for 5 wickets) declared 670 in 148.3 overs
FOW CONTINUED: 541
WORCESTERSHIRE SECONDS – SECOND INNINGS
Kohler-Cadmore lbw Bruce 9
Westbury b Fuller 34
Kervezee not out 65
Hepburn c White b Sowter 6
Malik not out 23
Total (for 3 wickets) 143 in 43 overs
FOW: 9, 78, 91