Annalise Murphy has won the two opening Laser Radial races to lead the class after day one.
Murphy won the first race by 15 seconds from Gintare Scheidt of Lithuania. She led the race from start to finish.
She then came from behind to win the second race. Belgian Evi Van Acker led at the first two marks, however, at that point Murphy assumed control, eventually winning by 45 seconds from the Belgian.
The two results mean that she leads the class after day one, with eight more races to come before the medal race.
Murphy admitted that she "couldn't really ask for much more" after marking her Olympic debut with victory in both races.
"It couldn't really get much better, but there is still a long regatta to go," Murphy said.
"I am just going to keep doing it one race at a time and see how things work out.
"I got a good start to the first race and was leading at the windward mark and led the rest of the way around.
"It was the perfect race and in the second Evi [Van Acker of Belgium] was a good bit ahead of me.
"I managed to catch up with her, got a few good little shifts on the second upwind and once I got ahead I pulled away quite a lot.
"I couldn't really ask for much more."
Ireland's Ryan Seaton and Matt McGovern finished fourth in the day's first 49er race, and eigth in the second race. Those results leave them in sixth place overall.
The first race was won the French crew of Emmanuel Dyen and Stephane Christidis in a time of 29 minutes five seconds. Seaton and McGovern came home thirty seconds later.
The second race was won by the Australian crew of Nathan Outteridge and Iain Jensen. They clocked a time of 28:08, Seaton and McGovern came home 1:01 later.
James Espey finished in 38th place in the first Laser Class race of the day, 2:50 behind the winner Juan Ignacio Maegli Aguero of Guatemala.
In the second race of the day he came home 44th, 4:06 behind Australian Tom Slingsby. Espey lies 42nd overall.
In the Star class, Peter O'Leary and David Burrows finished 14th in the third race of the competition.
New Zealand's Hamish Pepper and Jim Turner won the race in 1:11:19. O'Leary and Burrows were 1:31 adrift of that time.
In the fourth race, the Irish duo fared much better, coming home fifth. Sweden's Fredrik Loof and Max Salminen won the race in 1:11:54. O'Leary and Burrows crossed the line one minute later.
The Irish duo lie fifth overall.
Annalise commented "I am delighted with today but it’s only the first day. There is a lot of racing ahead.
"I had a good start and tacked across the beat and pulled away. It was great. I couldn't do much wrong today. I have got brilliant upwind speed. It's all coming together."
James O’Callaghan, ISA performance director remarked: “Annalise got off to a good solid start, but she still has five more days of racing ahead of her.
"This is her first Olympics and her competitors are well experienced with the Games. She needs to continue to focus as it’s still very early days”.
Commenting on the Star class, O'Callaghan stated: “The Star is a hugely competitive fleet as was seen by today’s racing.
"At the first mark all boats rounded within 60 seconds. This class is wide open, but a good performance by Peter and David."
And on the 49er class, O'Callaghan added: “This is a great start for the boys.
“They’ve a long road ahead with another 13 races to sail before the medal race but they’ve shown they can perform under pressure.”