The battle lines drawn, Ainslie went from fifth to second, to first after three legs of the second race only to see Scheidt, who was given a yellow card penalty in the day's first race, move from 13th to 12th, to second, six seconds behind him.
Displaying a coolness that belies his status of being, at 19, Britain's youngest ever Olympic sailing representative, Ainslie held off his 23- year-old rival and went into a weekend break with a five-point lead.
Monday sees the ninth and 10th races, Wednesday the final encounter but it means he needs only two more good results because the two worse results can be discarded from the 11 races.
Ainslie has already shrugged off the nightmare of a 27th in the first race. The rest run 4, 7, 2, 2, 1, 2, 1. He is 19 points clear of third- placed Peer Moberg, of Norway, and needs to score well, while beating Scheidt twice more, to take the gold.
Andy Beadsworth, Barry Parkin and Adrian Stead were over the line in their first Soling race of the day, went back to re-start, and played the shifty 10-knot westerly cleverly to work their way back up to finish seventh.
Shirley Robertson finished 22nd in the Europe but she was second in her second race to move nine points clear of the third-placed Courtney Becker-Dey, of the US.