Gloucestershire 317 and 135 for 5 (Harris 62) trail Northamptonshire 479 (Rickelton 95, Procter 78, Sales 71, Young 61, Gohar 5-134) by 27 runs
Unbeaten on 21 overnight, the 19-year-old shared a key ninth-wicket stand of 78 with Ben Sanderson as the visitors extended their first innings score from an overnight 353 for seven to 479 all out, a lead of 162.
Trailing by 36 runs at the start of the day, Gloucestershire needed to back their bowlers with tight fielding to restrict the deficit to a manageable level.
Instead, they dropped Sales twice when he had added only a single to his overnight score, David Payne the unlucky bowler on both occasions. First Harris spilled a chance two-handed to his right at gully. Then Ollie Price failed to capitalise on an edge to second slip.
Despite Zafar’s four wickets on day two, Gloucestershire bowled seam for the first 11 overs, only introducing their specialist spinner after 30 runs had been added. Three runs later he made the breakthrough, Simon Kerrigan, on 14, driving a catch to extra cover.
Sales moved to fifty with a pulled four off Zafar, having faced 121 balls and hit 6 fours. Sanderson then extended the lead to three figures before the pair took 12 off the only over of the innings bowled by Gloucestershire captain Graeme van Buuren.
By lunch, Northants had reached 454 for eight, having added 101 in the morning session. Sales’ vital contribution ended when he was unlucky to edge a leg side delivery from Zak Chappell through to James Bracey, whose wicketkeeping throughout the innings was exemplary.
A couple of sixes by last man Jack White off Zafar added to Gloucestershire’s frustration and Sanderson was only four short of his career best score when spooning a catch to cover off Chappell.
The home side had progressed their second innings to 37 when Chris Dent departed for 16 in careless fashion, chipping a catch to short mid-wicket off Taylor.
Ollie Price was caught behind off White for a seven-ball duck and at 42 for two, Gloucestershire trailed by 116.
Harris went on the counter-attack, lofting Kerrigan for two sixes over wide long-on as he and Miles Hammond took the total to 82 for 2 by tea in poor light.
With only a run added, Taylor beat Hammond’s defensive shot and trapped him lbw. Harris was beaten by successive balls from Sanderson, but then took a single off him to reach a 67-ball half-century, with two sixes and five fours.
Gloucestershire’s hopes of saving the game were dealt a severe blow when Harris advanced down the pitch to Kerrigan and was bowled between bat and pad.
James Bracey followed for 23, bowled by offspinner Rob Keogh’s first delivery of the innings, a full one which crept under the bat, and the hosts were in dire trouble at 132 for five.