In the other tie, Eastern Harbour beat Balmacewen Djokovic 4-2 while Balmacewen Federer had a bye, with the student team absent on holidays.
Mitchell Sizemore took St Clair's only point when he beat his long time rival Carlos Reid 6-3, 4-6, 6-1.
In their previous meetings this season, Reid had held match points before succumbing, while Sizemore had got within two points of winning before he, too, lost in a tight finish.
So the expectation of a close three-setter looked imminent only for Sizemore to rewrite the script and run away with the third set, which he dominated with power and accuracy.
The finish was summed up when he broke Reid to love at 4-1, at a point when comebacks had been a feature of their previous meetings.
One service break had been enough to decide the first two sets and Sizemore had generally been holding with greater ease in the second before he dropped a game on his delivery, and Reid held to take it to a decider.
Taieri's Darryl Paterson, Phil Williams and Graeme Clarke were all efficiency and held a margin over their rivals in the other positions although veterans Tony Ryder and Robin Versteeg claimed a set in their doubles loss.
Ryan Eggers (Eastern Harbour) continued his successful run with a 6-4, 6-4 win over Paddy Ou, but had to battle against a tenacious opponent, who twice fought back from 1-5 down in each set to get back to 4-5, before being ousted.
George Alexander picked one up for Balmacewen, beating Christiaan Faber 6-3, 1-6, 6-1 with the score telling the story. Alexander has always had the talent but has tended to show it in patches as his concentration has wavered.
He is transferring to Christchurch and will be a big loss for a club that has already had injury and transfer issues.
Harbour won the lower order singles but dropped the bottom double. Eggers and Faber took an entertaining and tight doubles against Ou and Alexander in a 10-7 match tiebreak.
Next weekend, the Otago A and B teams will travel to Queenstown and Oamaru for representative fixtures.
