Glamorgan 563 for 5 (Northeast 308*) trail Leicestershire 584 (Mulder 156, Mike 91, Hill 81, Kimber 68, Walker 64, Swindells 52, Salter 4-158) by 21 runs
Sam Northeast, seen by many as one of the best English players never to taste international cricket, notched the first double-century of his career and converted it to only the second triple-hundred in Glamorgan history as his side closed day three of their LV= Insurance County Championship match on 563 for 5 in reply to Leicestershire's 584.
The 32-year-old eclipsed his previous best of 191 for Kent against Derbyshire in 2016, finishing on 308 not out - just one short of matching Steve James's 309 not out against Sussex at Colwyn Bay in 2000 as the highest first-class score by a Glamorgan player.
He shared a 306-run partnership for the third wicket with Colin Ingram, who made 139, comfortably beating the Glamorgan record against Leicestershire of 242 set by Steve James and Matthew Maynard in 1995, and another of 229 - unbroken so far- for the sixth with Chris Cooke (71).
Northeast could have been out on 96 - missed at slip - while Cooke was dropped twice as Leicestershire, for whom Chris Wright took three for 74, missed a number of opportunities to take hold of a match that now looks likely to end in a draw.
"There were times when they bowled really well and you had to ride that," Northeast said. "I gave a chance on 96 but managed to survive that and battled for a bit. After that I began to feel it was a situation where I could get to 200 but 300 was never in my wildest imagination and a dream come true in a lot of ways.
"We were in a situation in the game where we needed to take time out of the game, to keep batting and get past the follow-on target, so that was my main priority but, yes, the double hundred was a thing that needed ticking off in my career. I've made some big hundreds before without getting a double so there was a bit of relief to get over the line. Once we had avoided the follow-on we were able to express ourselves and have a bit of fun at the end and the runs kept flowing.
"It was a strange year in 2021 after deciding to leave Hampshire and then playing on loan for two different counties but I feel I've found a real home at Glamorgan and I couldn't be happier."
Northeast and Ingram completed their centuries in a morning session in which both could have been out without reaching three figures, each in turn missed by the normally reliable Colin Ackermann in the slips.
Ingram, on 77, edged a ball from Ackermann's compatriot, Wiaan Mulder, that went between his legs at second slip, while Northeast should have gone to the ball that saw him go to 100 with his 16th boundary, an edge flashing past Ackermann's right shoulder as the only slip to spinner Callum Parkinson. On each occasion, Ackermann was left clasping thin air.
Ingram completed his century from 75 deliveries when he drove Ben Mike square of the wicket for his 14th boundary. Glamorgan were 242 for 2 at lunch.
Spinners Parkinson and Ackermann bowled for almost an hour in tandem at the start of the afternoon, without making any inroads, but when Mulder returned at the pavilion end, things began to happen.
Ingram almost played on to his fellow South African on 138, picking up only one run more before Mulder found just enough movement to induce an edge, Harry Swindells diving low to his left to take a good catch. The wicket broke the partnership at seven short of Glamorgan's third-wicket record against any opponent.
The new ball was available in the next over, which Leicestershire took immediately despite Mulder's success with the old one. The decision was vindicated when a testing five-over spell by Wright was rewarded with wickets from consecutive balls as Kiran Carlson was bowled off an inside edge, driving, before Billy Root edged to second slip, where Ackermann - no doubt to his own relief - took the catch.
Having grabbed three wickets for 29 in 10 overs, with Glamorgan still 101 runs away from avoiding the follow-on, Leicestershire sensed an opportunity, yet the session ended in more frustration for Mulder as Swindells dropped Cooke on three - a much easier chance than the one he had held earlier - which would have sent Glamorgan in at tea on 341 for 6.
Cooke escaped for a second time soon after tea, dropped by Louis Kimber at second slip on 15, again off the suffering Mulder. Having profited from Glamorgan's laxness in the field, Leicestershire seemed oddly determined to pay their opponents back in kind.
Glamorgan were only too pleased to accept their generosity, the next passage of play seeing them match Leicestershire with five batting bonus points shortly before Northeast reached the 200-mark with his 22nd boundary as he drove Parkinson handsomely down the ground.
The follow-on was avoided as the total passed 435 in the same over as the Northeast-Cooke partnership moved into three figures.
With Leicestershire's bowlers looking understandably weary, Northeast's third hundred came off just 74 balls, completed when he cut Ackermann for the 37th or his 38 fours so far.