Bates blazed 142 runs from just 126 balls, smacking 13 fours and a rare six in her second one-day ton of the season.
She has now amassed 571 runs at an average of 95.16 in Otago's first eight games of its title defence.
While the 56-run win was the fourth straight after opening the season with three losses, the Sparks will need to beat Wellington twice later in the month, and hope some other results go their way to make the final.
After winning the toss and electing to bat, Bates' latest innings helped the Sparks set an imposing 302 for eight at Molyneux Park.
Bates and Leigh Kasperek (74 runs) set an Otago record with a 214-run second-wicket partnership.
That blitzed the previous record set by Bates and Nicola Bolton of 133 against Wellington at Lincoln in the 2012-13 season.
While it fell 13 runs short of beating Debbie Hockley and Nicola Payne's 227-run, second-wicket partnership for Canterbury against Wellington in the 1999-2000 season, it lifted the Sparks from 26 for one to 240 for two.
Otago Sparks coach Warren Lees praised Bates and Kasperek for their efforts and said the team ''is on a wee bit of a roll''.
''They batted really, really well. Suzie [Bates] just continued her golden run, really. She's just getting further ahead of all the other players in the country.
''The rest of the team is also playing quite well. Abbot has got runs, Kasperek has got runs, Katey Martin has been pretty regular. We are pretty happy with how we are going,'' he said.
When Natalie Dodd dismissed Bates in the 41st over to leave the Sparks 245 for three, a total beyond 320 was on the cards.
However, the Spirit bowlers snared five more wickets and only Katey Martin (23) and Victoria Abbot (12) managed to score double-figures at the death.
Almost all the Northern bowlers took some tap, but Brooke Halliday was the pick of the lot, with three wickets for 46 runs from her eight overs.
In the field, Bates was quick to make her presence felt with ball in hand, removing opener Caitlin Gurrey lbw with just the second ball of Northern's chase.
She then snared Beth Mooney for 12 to reduce the Spirit to 20 for two after just 2.2 overs.
At 106 for five after 18.2 overs, Northern looked dead and buried. But Dodd and Felicity Leydon-Davis put on 86 for the sixth wicket to give their side a sniff.
However, the Sparks kept their cool and Northern could only manage 246 for nine in reply. Dodd top-scored with 118 from just 99 balls, while Leydon-Davis struck 53.
Abbot was the pick of the Otago bowlers, taking three for 42.
In yesterday's other games, the Auckland Hearts beat the Wellington Blaze by seven wickets and the Canterbury Magicians thumped Central Hinds by 158 runs.
In Saturday's game at Molyneux Park, the Sparks had no trouble chasing down Northern's lacklustre total of 203 all out.
No 3 Kasperek led all Otago scorers with an unbeaten 57, while Bates (55), Abbot (48) and Martin (27) also contributed in the convincing seven-wicket win.
The pick of the bowlers was Kasperek, who finished with two for 32 from her 10 allotted overs, while a combination of poor running and sharp fielding resulted in five Northern players being run out.
Coach Lees said he could not recall another game in which five players in one innings had been run out, and described his team's fielding performance as ''probably the most complete'' of the season.
By Robert van Royen.
