But the free-scoring Hancox was forced to retire hurt with a hamstring injury after scoring 21 off 25 balls.
Thornley kept up the attack, with some quality shots especially through the on-side, and brought up his half-century before he fell with the score at 100/3.
Leeston-Southbridge's fightback was sparked by Mainpower Oval head groundsman Dave Evans whose nagging medium-pacers turned the match, picking up three quick wickets.
The spin trio of David Neal, Tim Gruijters and Will Macfarlane, also combined for five wickets and Southbrook were bowled out for a disappointing 150 in 37.5 overs.
Southbrook needed to strike early if they were to have any chance in defending the low total, and through paceman Jono Green they got their wish.
He ripped out opener Rupert Young and Gruijters in the first over, giving the desperate Brook a sniff of a rare victory.
However, that hope was slowly eroded by the patient and composed batting of Toby Doyle and Chris Beatson.
Unperturbed at taking 39 balls to get off the mark, Doyle in particular ground Southbrook out of the match.
He made an unbeaten 45 runs off 117 balls, while Beatson was more free-flowing, if no less measured, in compiling 66 not out from 98 balls, securing the eight-wicket win with eight overs to spare.
An unbeaten century by New Zealand Under-19 star Rhys Mariu helped guide Ohoka to a hard-fought 15-run win over Darfield.
Mariu carried his bat through the first innings, making 114 of Ohoka's 253 runs, well-supported by Will Hamilton (41 off 46) and Harry Fitzpatrick (46 off 66).
In reply, Darfield's reply got off to a fine start and were sitting pretty at 92/1 in 19 overs, before stuttering to be 117/5.
However, Ryan Hughes (38 off 40) and 'keeper Alister Collins (48 off 45) gave Darfield a whiff of victory but ultimately fell 15 runs short.
At Weedons Domain, visiting Cheviot thumped the home side by 115 runs thanks to an overall team display.
A century partnership between Cheviot opener Ted Whelan (65 off 74) and George Waddy (68 off 97) set up the commanding first innings score of 251 in their allotted 45 overs.
Weedons never threatened the big score, losing wickets consistently to be all out for 136 in the 37th over. Right-arm quick Will Smith was the main destroyer, bagging three wickets.
At Pearson Park, Oxford-Rangiora powered to a five-wicket win over struggling Lincoln after rolling the visitors for just 102.
Calvin Scott again led the run chase, anchoring the innings with 35 off 61 balls and helping Oxford-Rangiora to complete the victory with 19 overs to spare.
Sefton had the bye.
Short scorecards:
• Southbrook 150/9 (H Thornley 58, T Hancox 21 rtd hrt, K Bayer 18; W Macfarlane 3/16, D Evans 3/18)
Lost to Leeston-Southbridge 151/2 (C Beatson 66no, T Doyle 45no; J Green 2/37).
• Ohoka 253/6 (R Mariu 114, H Fitzpatrick 46, W Hamilton 41; J Frew 3/53)
Beat Darfield 238/9 (A Collins 48, N Gilbert 39, R Hughes 38, J Frew 31; G Mauger 4/41).
• Cheviot 251 (G Waddy 68, T Whelan 65, C Burnett 39; S Clarke 4/47, D Nightingale 2/31, H D'Arcy 2/39)
Beat Weedons 136 (B Agnew 27, S Clarke 26, B Nightingale 20; W Smith 3/23, A Redfern 2/6, S Burnett 2/27).
• Lincoln 102 (H Paterson 25; F Fairbairn 4/18, M Murphy 3/20)
Lost to Oxford-Rangiora 103/5 (C Scott 35, L Stove 18; A Gulati 2/23, E Paterson 2/25).
