Auckland's late NPC rugby slip-up against Canterbury became even more significant when the chasing group all won to close the gap on the former titleholders.
Otago have used their substitutes bench and get out of jail cards in the last two rounds to join Auckland in the series leadand set up an appetising meeting at Carisbrook on Saturday.
The big quartet - Auckland, Otago, Canterbury and Waikato - have a useful break on the field with four rounds to go until the playoffs.
Defending champions Otago need all the fortune which has come their way because they have the most difficult run home to make the final four, with games against Auckland, Canterbury, Waikato and Wellington.
The other favoured sides all have scheduled matches against teams in the lower half of the competition.
As Auckland coughed up their outright series lead to a Daryl Lilley drop goal for Canterbury, Otago then Waikato yesterday ground out victories to keep them in the top four.
In the slipstream are Wellington, the yo-yo side who beat Hurricanes partners Taranaki and probably need an upset Ranfurly Shield win this weekend against Waikato to stay in touch for the semifinals.
For a while yesterday an upset appeared possible in Invercargill as Southland gave Waikato a hurry-up. As expected, they took on the visiting pack and with a little more finesse out wide could have produced their first win of the year.
Waikato coach Kiwi Searancke sounded a note of warning.
"If we play like that next Saturday, then we won't have the Shield," he said. "We were frustrated with our performance, in fact it was all a bit disappointing.
"I don't want to take anything away from Southland, but ours wasn't a great performance. I know we had several changes and substitutions but there should be enough depth in our squad to overcome that."
Southland remain the only firstdivision side without a win after North Harbour broke their 1999 drought by beating franchise partners Northland.
That result means Counties Manukau, North Harbour, Northland and Taranaki all have just one victory, while Otago and Waikato, with a draw and four wins, are the only unbeaten teams.
