Derbyshire 274 for 4 (Masood 113, Guest 77) trail Worcestershire 368 (Haynes 133, Ali 88,Conners 5-109) by 94 runs
The Pakistan left-hander made 113 from 132 balls to take his tally to 826 with a possible three more innings to come before the end of the month.
The day had promised much more for the visitors but after subsiding to Conners they felt the full measure of Masood’s elegant strokeplay.
Ed Barnard and Ben Cox represented Worcestershire’s best chance of passing 400 but they both fell in the same over to Conners, Barnard driving loosely to cover and Cox lbw to a ball that looked to be missing leg stump.
Conners made it three in seven balls when he found lift and away movement to remove Josh Baker before nightwatchman Adam Finch, after 22 overs of determined resistance, pulled Ryan Sidebottom to deep backward square.
Sidebottom immediately left the field with a calf injury but Anuj Dal pulled off another stunning catch at point to quickly end the innings and give Conners his fifth wicket.
Derbyshire set the tone for what followed by scoring 30 in seven overs before lunch and the afternoon became one to remember for Masood and one to forget for Worcestershire.
Although they are missing three frontline bowlers through injury, there was no mitigation for the way runs were leaked on both sides of the wickets with the bowlers unable to exert any pressure.
After Billy Godelman got an inside edge onto his leg stump, Masood and Guest cruised along at almost a run-a-ball against indisciplined bowling and increasingly ragged fielding.
Masood’s timing and placement was exemplary but Worcestershire helped him along the way, Barnard spilling a chance at backward point off Ben Gibbon on 67 and then fluffing a run out chance 10 runs later.
Inevitably, he moved to his third century in seven innings from 115 balls but Worcestershire regrouped after tea with Gibbon claiming the prized wicket of Masood during an impressive 11-over spell from the City End.
The 21-year-old beat Masood’s attempted drive in the first over after the interval, the first of three wickets to fall for only 27 runs in 15 overs which reflected Worcestershire’s improvement.
Guest lost his concentration and edged a big drive at a wide ball from Barnard and Wayne Madsen was lbw trying to work Baker’s left-arm spin to leg.
Gibbons was unlucky not to make further inroads but Leus du Plooy and Luis Reece saw Derbyshire through to the close although Worcestershire have the prospect of a second new ball early on day three.