Durham 364 for 4 (Lees 171, Bedingham 138) track derbyshire 443 (Guest 197, Madsen 62, Borthwick 4-25) for 79 runs
The pair added 305 from 333 balls for the third wicket, Durham’s best, and at stumps on the second day the visitors were 364 for 4, some 79 runs behind Derbyshire’s 443 first innings.
Durham had previously been on the receiving end on a flat bowl when Guest and tail took advantage of some poor bowling. Derbyshire added 126 runs from 25 overs with Guest and Conners doing most of the damage as they looted 71 off 48 balls.
Guest got to his 150 by converting Ben Raine behind square for his 23 four before the Durham closer got one to straighten and knock out Alex Thomson’s stump.
Zak Chappell slotted in a short leg lifter from Brydon Carse, but Durham lost control once Guest and Conners engaged the turbo.
Conners took Carse out for six and after Derbyshire collected five penalty runs as the ball hit the helmet behind goalkeeper Ollie Robinson, he pushed Matthew Parkinson over the long limit.
Durham looked increasingly patchy until Conners charged Scott Borthwick and was stunned for 33 from just 25 balls.
Guest had a double century in his sights, but when he came up against Borthwick, he was unable to clear Raine for long.
It had been an outstanding inning and it was made even better when Conners hit twice in the fourth after lunch.
Michael Jones was brought down as he prepared to play through the midwicket and Borthwick was caught in the crease, but Lees and Bedingham responded with a thrilling counter-attack.
Bedingham lit the fuse by taking out Chappell for six before Lees drove straight past Thomson for six in a row.
Lees drove Anuj Dal back over his head for another six and his only moment of anxiety came in the 79th when he nearly overtook Chappell from behind.
But he drove the next ball for four and then turned Chappell into a single to reach that 1,000 milestone which was quickly followed by his fourth century in five innings when he hammered Dal to the fine leg limit.
Lees had faced just 127 balls, but Bedingham hit his hundred off just 113 when he edged past Luis Reece to third man for his 15th four.
The runs kept coming and the pair passed Durham’s previous third-highest wicket position of 243 which they had established at Leicester in late June.
Lees went to his 150 with four successive off Nic Potts, but the position was finally broken when Bedingham clipped Dal to substitute for fielder Mitch Wagstaff at short half-wicket.
Durham sent in Migael Pretorius to take advantage of a tiring attack, but Conners came back to remove Lees, who was caught from behind and two overs before the close.