The five-time world champion won both of his opening races to take an early lead in the Finn class. Ainslie led throughout the first race in the yellow fleet and came from fifth to win the second race from compatriot and current world champion Ed Wright.
"I didn't have a particularly good start [in the second race], but I was good on the downwind reach and was in front at the second mark,'' Ainslie said.
Sara Winther was the pick of the Kiwis in the Laser Radial fleet with a seventh and a sixth to be seventh overall but Natalia Kosinska had a difficult day in the women's RS:X with a sixth and 24th to be 28th overall.
The world championships are the biggest event on the sailing calendar this year and double as the Olympic qualifiers, with 75 per cent of the country qualifications for London decided at this regatta.
Snow-Hansen and Saunders will be hopeful of qualifying the men's 470 with the two 21-year-olds already well placed and they took advantage in the first race of the day in variable conditions.
"The first race was still a building sea breeze,'' Saunders said. "It was really patchy and shifty. We were able to take a look at the first flight that started ahead of us, and we learned a bit from that. We just sailed conservatively, and it turned out pretty well.''
The duo followed this up with another top-10 finish in the second race. By this time the Fremantle Doctor had arrived with some stronger pressure.
"It was a good day, a solid start to the regatta. Tough conditions to race in though'' Saunders said.
The world championships are being contested be more than 1200 sailors from 80 nations.
They run until December 18, with women's RS:X and Laser Radial and men's 470 and Finn in the opening week and men's RS:X, Laser, 49er and Star and women's 470 in the second week of competition.
New Zealand's current standings:
Men's 470
5th: Paul Snow-Hansen and Jason Saunders (1, 9)
59th: Luke Stevenson and Sam Bullock (33, 26)
61st: Mike Snow-Hansen and Derek Snow (24, 36)
71st: James Turner and Matthew Jones (37, 31)
73rd: Gareth Moore and Ben Goodwin (35, 35)
Finn
13th: Dan Slater (5, 11)
30th: Matt Coutts (15, 15)
69th: Rob Coutts (35, 34)
51st: Brad Douglas (24, 26)
53rd: Nik Burfoot (26, 25)
Women's RS:X
28th: Natalia Kosinska (6, 24)
39th: Justina Sellers (16, 22)
49th: Steff Williams (26, 21)
Laser Radial
7th: Sara Winther (7, 6)
45th: Rachel Basevi (3, 42)
63rd: Miranda Powrie (26, 33)
Women's match racing
Stephanie Hazard, Jenna Hansen, Susannah Pyatt (4 wins, 1 loss)