Tunnicliffe secured the gold in front of a packed grandstand with four straight wins, despite some very tight sailing from Macgregor who was beset by penalties through out the afternoon.
"It's amazing, we've put a lot of effort into this year and had a lot of big events we had to get through and this has really topped off all our hard training," Tunnicliffe said. "The team sailed really well, it was tricky conditions, conditions that we actually thought we were weakest in so we're psyched we came out on top."
"It was so close, neither team could give up. Everyone was pushing it to the limit," Tunnicliffe said. "In the last race it was an inch or two to the finish line. That pretty much summed up the racing."
It was a game of cat and mouse between the two teams contending the world title and four races of incredibly close and highly skillful racing, drew gasps and cheers from the crowd throughout. But it was Team USA who had the biggest cheers at the end of the contest and MacGregor was left to rue the penalties she incurred that put her on the back foot.
In the battle for third and fourth place it was France's Claire Leroy who came out on top to take the bronze medal with a 3-1 victory over Ekaterina Skudina (RUS), with Leroy's superior downwind speed and tactics making the difference on the day.
In the Stars, defending world champions Robert Scheidt and Bruno Prada (BRA) look set to retain their World title with an 18 point overall lead going into Saturday's Medal Race.
As the rest of the fleet fought for a place in the top 10, Scheidt and Prada placed a second and third to end the day with 35 points overall after 10 races.
The top three teams remain unchanged from Thursday, with Americans Mark Mendelblatt and Brian Fatih still in second place on 53 points, and German pair Robert Stanjek and Frithjof Kleen only two points behind in third.
Despite retaining their second place, the Americans had a disappointing day on the water, placing eighth and tenth in Races 9 and 10.
Women's 470 fleet leaders Tara Pacheco and Berta Betanzos (ESP) consolidated their lead today and are ahead by a solid 13 points going into Saturday's final.
Japanese duo Ai Kondo and Wakako Tabata, on 44 points, have some serious catching up to do for a chance at winning the Worlds and will need to perform strongly on Saturday. Three points behind are Gil Cohen and Vered Buskila (ISR) in third.
With only two races to go and a fleet overall placing of 15, World #1, Erin Maxwell and Isabelle Kinsolving (USA), have an almost impossible task ahead of them to secure a place in the top 10 fleet for Sunday's Medal Race.
The Women's 470 competition is scheduled to continue from 14:30 local time on on Saturday, with the top 10 going through to contest the world championship in the Medal Races on Sunday.
In the 49er John Pink and Rick Peacock (GBR) held onto their first place overall and are just two points clear as the 49ers head into Saturday's final day of Gold fleet racing. The British pair are on 61 points, Nathan Outteridge and Iain Jensen (AUS) are on 63 with the Toft Neilsen brothers (DEN) on 70. Two points behind them is the New Zealand pair, Peter Burling and Blair Tuke.
The 49ers Gold fleet has three races scheduled for Saturday, before the top ten go into to the final medal race to decide who will the 2011 ISAF Sailing World Champion.
The Perth 2011 ISAF Sailing World Championships website will bring you results, tracking, live updates and all the latest news both on and off the water.