The United four of Dale Bourke, Dianne Potts, Misty Arnold and Barbara McGregor trailed 14-7 after 15 of the 18 ends against Dorothy Burnard (Green Island) when they scored a seven on the next end to draw level and two more on the next end to take the lead.
Green Island came back to score two and force the extra end, but the Nelson team was too strong and scored five shots to win the game 21-16.
McGregor also won the quarterfinal 22-17 when she beat Sandra Keith's Allenton four from Ashburton.
Her four was trailing 11-8 after eight ends when it scored a six on the ninth end to take back the lead.
"Those two shots made a big difference when the games were slipping away from us," McGregor said.
The quarterfinal was interrupted for 40 minutes during the severe rain storm when the green was flooded and hail fell on the green.
But conditions remained calm after this and the St Clair green was in tip-top condition after the surface water was swept off.
The United team scored 11 shots from the eighth to the 11th ends to lead 18-11. But it was never able to relax because Allenton kept coming back at it and closed the gap to 18-16 after 14 ends.
The Boyd sisters, Mandy (19) and Angela (24), made a significant contribution to the 23-18 win by Sue Burnand's Frankton Railway team that beat Wynette McLachlan's South Otago combination 23-18.
In the men's competition, Gary Lawson (Eastbourne), the favourite for the title, beat Greg Ruaporo (Blockhouse Bay) 24-9, Kerry Becks (Kaiapoi) eliminated Mark Watt (North East Valley) 19-14, veteran Peter Sain (Onehunga) beat Ryan Khan (Johnsonville) 17-11 and Ian Brown (North End) beat Andrew Kelly (Canterbury) 17-11.
The big surprise of the day came when the star-studded Lawson team was beaten by former international Terry Scott (North East Valley) 21-13 in the second round and Sain was beaten 17-16 when Brown drew a shot close to the jack in the ditch with his last bowl.
Brown advanced to the last eight when he beat Gary Andrews (St Clair) 18-8 in his third game.
The North East Valley team of Steve O'Driscoll, Jim Scott, Shaun Scott and Terry Scott reached the last eight when they beat Trevor Pascoe, Dennis Leeden, Lance Pascoe and Ken Walker (Fairfield) 18-15 in a cliffhanger.
The scores were even at 13-13 in the Scott-Lawson game after 14 of the 18 ends. The turning point came in the next two ends when North East Valley scored seven shots.
Shaun Scott, who won the singles on Tuesday, drove the jack into the ditch for four shots on the 15th end.
On the next end, Terry Scott ran the jack into the ditch for three shots with his last bowl when Lawson held three shots on the head.
North East Valley made a slow start and trailed Walker 7-1 after five ends when Scott came back into contention with a five on the sixth end.
"It was big for us," Terry Scott said. "We had five shots on the head on two other occasions before Ken cut the count down."
Both teams were tired after an 11-hour day on the green in trying conditions. The wind in the last game was cold and blustery and made accurate draw bowls difficult.
The North East Valley team held a one shot advantage before the final end. It looked hopeful for Walker's team when Leeden ran the jack into the ditch. But first Shaun Scott drew the shot close to the ditch and Terry Scott added the second shot.
