Olivia Price's Australia overcame Finland and France, while the Netherlands and Sweden lost to the Ekaterina Skudina-skippered Russian team.
Spain were the other Elliott 6m crew to finish the day with a 100 per cent record, overcoming the much-fancied British and USA teams.
The conditions were perfect. We won two races so we certainly enjoyed today's sailingAngela Pumariega Menendez
'It was a good day,' said Angela Pumariega Menendez, part of the three-woman keelboat alongside skipper Tamara Echegoyen Dominguez and Sofia Toro Prieto Puga.
'The conditions were perfect. We won two races so we certainly enjoyed today's sailing.
'For the moment, we are sailing good in each race. It is a good day for us.
'We felt a little nervous in the first race as it was our first race of the Olympics.
'But we tried to concentrate and sail in each gust because the wind was very shifty.
'It was against the British girls so we had a lot of pressure, but we concentrated and tried to do our best.'
2010 world champions Great Britain ended the day with defeat to Spain, having beaten Denmark earlier in the afternoon.
'We can definitely take confidence from today,' said Annie Lush, who races alongside sisters Kate and Lucy Macgregor.