That McCaul and Shirley could battle to make the final was evident in the quarter-final, when they just beat Mangere's Moana Stainton and Paddy Waerehu.
There was better news in the men's pairs for members of the New Zealand squad who have been using the Open as preparation for the Asia-Pacific Games later this month.
Richard Girvan and Danny Delany qualified for tomorrow's final even though they were pushed hard by a couple of Auckland veterans, Wally Marsic and Tony Garelja, winning 12-11.
The other finalists will be Canterbury's Alvin Gardiner who teamed up with Australian Alan Hilton to win their semifinal 14-11 over Wellington's Rob Ashton and his Auckland lead, Barclay Lee.
Black Jacks were in more commanding form in yesterday's early rounds of the men's triples, which saw the departure of two of last year's winning combination, Steve Beel and Lance Tasker, after their Bay of Plenty line-up was well beaten by a Jamie Hill-skipped Auckland trio.
Beel and Tasker lost their lead from last year, Tony Grantham, when he was chosen for the Black Jacks and yesterday Grantham's new partners, Girvan and Ali Forsyth, were in impressive form as was the other national line-up of Shannon McIlory, Chris Le Lievre and Delaney.