Denly and Key set Glamorgan a mountain
Glamorgan 207 (Riley 4-47) and 101 for 2 (Kettleborough 52*) need another 453 runs to beat Kent 309 (Key 94) and 451 for 2 dec (Key 158, Denly 161*)
Kent's hopes of making further inroads into Glamorgan's second innings and clinching their fourth LV= County Championship win of the season were held up by bad light in Cardiff.
The visitors had set the hosts the mammoth task of scoring 554 to win, and they were 101 for 2 at tea - with 32 overs still scheduled - but there was no further play on day three due to the conditions at Sophia Gardens.
Opener James Kettleborough (52 not out) and Colin Ingram (42no) shared an unbroken 78-run partnership for the third wicket.
Earlier, Glamorgan opener Jeremy Lawlor recorded a pair on his championship debut when he was bowled by Matt Coles, who then dismissed David Lloyd three overs later - the Kent seamer taking his season's haul to 65 wickets with one championship game left.
Kettleborough and Ingram then settled into their partnership, and with the pitch offering little to the bowlers they were rarely troubled by Kent attack.
Kettleborough, who has not played a championship match since being left out after playing the first five games, attacked with relish, striking 11 fours in his 74-ball fifty.
Kent had resumed on their overnight score of 256 for 1, with Robert Key and Joe Denly scoring quickly against a tiring attack.
Key, who notched a century in the previous match against Lancashire and 94 in the first innings here, reached 158 in the second before he was caught on the square-leg boundary by Graham Wagg off the bowling of Kettleborough.
It was the 54th hundred of his career, and, after returning to open the innings after batting at number three earlier in the campaign, he has scored 590 runs at an average of 73.
The 222-run stand between Key and Denly was a new record for the second wicket for Kent against Glamorgan - surpassing the previous record of 184 between Trevor Ward and Neil Taylor at Maidstone in 1991.
Denly remained undefeated on 167 - his third century of the summer - and shared the third three-figure partnership of the innings (114) with Sam Northeast, who made an unbeaten 50.
Northeast might have been better advised to have declared at lunch when Kent led by 496, which would have given his bowlers more time to dismiss Glamorgan, but he decided to bat on and score another 55 runs in six overs before calling a halt.