Stoughton, who entered the ninth and final draw tied atop the round robin with fellow Winnipeg native Mike McEwen, defeated McEwen 7-5 in 10 ends to secure first place on the men's side with a 5-1 record.
The loss was McEwen's second of the day after falling 7-4 to Calgary's Kevin Koe in the afternoon draw at Mosaic Place. McEwen (4-2) will now play a rematch against Koe (4-2) in a tiebreaker Saturday after Koe beat Edmonton's Kevin Martin 6-3.
The winner will face world champion Glenn Howard of Coldwater, Ont., in the semifinal. Howard (4-2) secured the spot with a 7-3 victory over winless Brad Gushue of St. John's, N.L.
It was a disappointing tournament for Martin, the defending Olympic and Canada Cup champion, who is going home with a paltry 1-5 record.
On the women's side, Saskatoon's Lawton locked up the top spot in the round robin despite losing 4-3 to Winnipeg's Cathy Overton-Clapham (1-5). Lawton topped the round robin with a 5-1 record.
Saturday's women's semifinal was decided in the final draw.
Team Jennifer Jones will play Heather Nedohin of Sherwood Park, Alta., in the semi after both teams won their final matches to finish with 4-2 records.
Team Jones advanced with a 7-5 victory over Winnipeg's Chelsea Carey (2-4). Kaitlyn Lawes is handling skip duties for Jones's Winnipeg rink since Jones gave birth to a baby girl two weeks ago and is also recovering from knee surgery.
Nedohin, meanwhile, needed 11 ends to beat Overton-Clapham 10-8.
Both the men's and women's finals are Sunday. Each winner will receive one of the six automatic qualification spots for the 2013 Roar of the Rings — the tournament that will determine Canada's entries at the 2014 Winter Games in Sochi, Russia.