Postma took the race win with Ainslie second and third place in Race 9 was taken by Giles Scott (GBR).
Ainslie was then black-flagged at the start of Race 10. The race was won by Rafael Trujillo (ESP) in a return to form, with Zach Railey (USA) second and Jonas Hoeg-Christensen (DEN) third.
In the final fleet rankings ahead of the gold medal race, Ainslie leads countryman Giles Scott and Dutchman Pieter-Jan Postma.
The Medal Race for the Finn class is scheduled to start at 16:10 local time on Centre Course
Defending world champions, Mathew Belcher and Malcolm Page (AUS), did enough on the penultimate day of the Men's 470 to put them in the box seat in Sunday's Medal Race off Bathers Bay.
After a busy night for the Men's 470 sailors in the Jury Room Gideon Kliger and Eran Sela (ISR) are back in the mix with only Belcher and Page guaranteed a medal. Great Britain's Luke Patience and Stuart Bithell are second on 38 points, and Croatia's Sime Fantela and Igor Marenic, are third on 46 points.
With double points on offer on the final day, the British would need to win and the Australians finish last in the 10-boat medal fleet, something that might be unusual given the Aussies have won six races and finished no lower than sixth of the nine counting races.
The Croatian pair had a stellar day, winning the ninth race and taking third in the 10th while Great Britain's Luke Patience finished eighth and ninth to slip six crucial points behind the Australians going into the Medal Race.
Two of the biggest movers in the final two races were Israel's Gideon Kliger and Eran Sela and Greece's Panagiotis Kampouridis and Efstathios Papadopoulos.
The Israelis finished fifth and first to go from 17th to a medal-race sixth before the jury hearing and were moved up to fourth late into the evening.
The Men's 470 Medal Race is scheduled to start at 14:10 on the Centre Course on Sunday.
The competition between Israel's Lee Korzits (ISR), Marina Alabau (ESP) and Zofia Noceti-Klepacka (POL) went up a notch in the RS:X on Saturday with the three leading sailors even closer on points after the final two Gold fleet races.
Just eight points separate the three athletes, ensuring each is guaranteed a medal as well as a place in RS:X history.
Korzits placed fourth and third in the final Gold fleet race but remains on top overall, closely followed by Alabau with Noceti-Klepacka third.
Despite Korzit's overall lead, Saturday's racing belonged to Noceti-Klepacka, the Polish sailor won both Race 9 and 10. The solid sea breeze also helped Olha Maslivets (UKR), who finished third.
The women's RS:X Medal Race is scheduled to begin at 15:10 on Sunday on the Centre Course.
On the eve of the all-important Medal Race, World #1 Marit Bouwmeester (NED) overtook Evi van Acker (BEL) to take the overall lead by six points with Paige Railey (USA) third on 53 points.
Annalise Murphy (IRL) moves up to fourth place despite dominating Saturday's Gold fleet racing with two wins.
Race 9 was delayed initially, but kicked off when the sea breeze settled in at 12 to 18 knots. It later strengthened to 17-20 knots, building up waves and swell, perfect downwind conditions for the Laser Radials.
Murphy sailed her own race, leading from start to finish to win Race 9 without much competition from the rest of the fleet.
Bouwmeester came back strongly to finish seventh after rounding the top mark in 31st place on the Centre course.
Bouwmeester's consistency saw her take a fifth finish in the final race of the day - securing her the overall points lead - while former competition leader Evi van Acker (BEL) suffered her worst result for the second time, a 19th.
Sunday's Medal Race for Laser Radial is scheduled to begin on Centre Course at 13:10.
Women's Match Racing
The repechage component of Women's Match Racing began on Fremantle's Inner Harbour Saturday.
With a fairly consistent sea breeze blowing in the afternoon, nine flights were completed on the first day of repechage competition, which will determine the final list for the Quarter Finals.
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.