British duo Percy and Simpson head into Sunday's medal race well placed to retain their Star class gold.
The Olympic champions from Beijing 2008 have an eight-point lead over second-place Robert Scheidt and Bruno Prada after the 10 opening series races.
The Brazilian pair did, though, win the first race of Day 7 as Simpson and Percy finished fourth, although the Brits responded superbly to get a victory in the second race of the afternoon.
Scheidt and Prada, with Sweden's Fredrik Loof and Max Salminen are the only ones able to take gold away from the British pair, who cannot wait for Sunday's medal race
'It was a good comeback and then a good response in the second race,' said Percy, who also has Finn gold from Sydney 2000..
'We lost points on our rivals in the first one and came back fighting so we are pleased about that.
'It is going to be a tight one as eight points is only three places in the medal race, really.
'It doesn't count for much so it is almost going to be a one-race series between three boats, but what a great occasion. What a great thing to look forward to.
'The medals are decided already, just not what colour. Because of that we will just be focusing on trying to beat those two in the race.'
The medals are decided already, just not what colour. Because of that we will just be focusing on trying to beat those two in the race.Iain Percy
The Finn class is shaping up to produce a dramatic battle, with Ben Ainslie's bid for a fourth Olympic Games gold in his own hands as only two points separate him and Danish leader Jonas Hogh-Christensen.
Elsewhere, Hannah Mills and Saskia Clark lead the women's 470 class after the opening day of action.
The pair recovered from a penalty turn to finish race one in sixth, before getting a victory later in the afternoon.
Luke Patience and Stuart Bithell also maintain their lead of the men's 470 fleet with a fourth and second today, while Belgium's Evi van Acker leads overall in the Laser Radial after an eighth and first today, although her rivals are close behind.
Things started badly for Australia's Laser fleet leader Tom Slingsby, but he recovered from a 15th in the first race to get a victory in the second race.
The Australians are also leading the 49er fleet, with Nathan Outteridge and Iain Jensen nine points clear of New Zealand duo Peter Burling and Blair Tuke.