The Force picked up their first ever win in New Zealand by beating the Blues at home by 17 points to 27.
The Blues, with three victories from their first four matches, were expected to beat the fast improving Force, although the loss of their key play-maker Nick Evans midway through the first spell was a telling blow.
After going into the break with a 17-7 lead, the Blues gradually lost the ascendancy as the Force clawed their way back into the match with 20 unanswered second half points.
The Perth-based Force was led by Giteau -- who contributed 12 points with the boot and brilliantly organised the backline -- and captain Nathan Sharpe who marshalled a vastly improved second half performance by the pack.
"All the boys put a rocket up themselves at half time and you saw the result in the second half," Sharpe said later.
"It's a fantastic result for my team," he said of the side's first win on New Zealand soil since joining the expanded Super 14 competition in 2006.
The Blues made the most of their early dominance of possession with a try to All Black prop Tony Woodcock who emerged from a breakdown just short of the Force tryline to barge his way over for a converted try.
Fullback Cameron Shepherd answered for the Force 14 minutes later, evading three tackles in a move that also featured Giteau and winger Drew Mitchell, another prominent figure in the winning side.
The loss of flyhalf Evans with a jaw injury midway through the spell and the brief loss of halfback Danny Lee to the blood bin appeared to unsettle the Blues as they rejigged their backline.
But they refound their shape in the last five minutes with fullback Ben Atiga kicking a penalty, and winger Anthony Tuitavake touching down for a converted try to give the Blues a 10-point lead in the dying minutes of the first spell.
The Blues started the second half with a greater sense of urgency than their opponents but the loss of playmaker Evans gradually took its toll as the Force showed increasing self-belief.
Relentless pressure by the Force with clean recycling of the ball at the breakdown was rewarded when Giteau threw a long cut out pass to No.8 Richard Brown for a crucial try in the 62nd minute.
Victory seemed all but assured for the Force after 69 minutes with a try to reserve Tamaiti Horua, who found a gap in the flagging Blues defence.
Pen - Ben Atiga.
Pen - Matt Giteau (2)
Venue - North Harbour
Referee: Craig Joubert (SA)
Ref's Asst. : Christie du Preez (SA), Paul Honiss (NZ)
TMO: Ben Skeen (NZ)
Assessor: Alan Riley (NZ)
1 Tony Woodcock , 2 Keven Mealamu ,3 John Afoa, 4 Anthony Boric , 5 Troy Flavell ©, , 6 Jerome Kaino ,7 Justin Collins, 8 Nick Williams , 9 Danny Lee , 10 Nick Evans ,11 Joe Rokocoko ,12 Benson Stanley, 13 Isaia Toeava ,14 Anthony Tuitavake ,15 Ben Atiga
Reserves: 16. Tom McCartney , 17. Nick White , 18. Kurtis Haiu , 19. Onosa’,i Tololima-Auva’,a , 20. Taniela Moa , 21. Rudi Wulf , 22. George Pisi
Cameron Shepherd, Scott Staniforth, Ryan Cross, Lachlan MacKay, Drew Mitchell, Matt Giteau, James Stannard, Richard Brown, David Pocock, Scott Fava, Nathan Sharpe (capt), Tom Hockings, AJ Whalley, Tai McIsaac, Troy Takiari.
Reserves: Luke Holmes, Pek Cowan, Sam Wykes, Tamaiti Horua, Chris O'Young, Scott Daruda, Nick Cummins.