"When you're playing good matches in big tournaments like this, I don't think it's peaking too soon".
With the win Stephens advances to her second consecutive Rogers Cup semifinal after falling to Caroline Wozniacki previous year.
The French Open champion showed no signs of fatigue following her marathon three-set win over Anastasia Pavlyuchenkova on Wednesday, running down balls to extend points and wear down the slower Garcia.
Barty will play the Romanian for the first time after dispatching Kiki Bertens 6-3 6-1 in less than an hour to extend her ideal record over the Dutchwoman to two matches.
American third seed Stephens surged into Sunday's final against world number one Halep after overpowering Svitolina 6-3, 6-3 in the day's second semi-final.
"I was pretty close [to winning] the last time so I'm just going to go out there and play hard", Stephens, who trails Halep 6-2 in head-to-head contests, said. Obviously, to make a final of another tournament is incredible.
"I think I talked enough about it", she said.
"I just went all the way also on her forehand, short, cross, long, everything".
Svitolina rallied briefly to get back to 5-3 but then handed Stephens another break and the first set when she sent a forehand wide.
After a shaky start, the French Open champion won three straight games to take the first set and dominated the second set behind pinpoint serving and stout defence.
She then raced through the second set 6-1 with minimum fuss.
Barty - who faced Halep for the first time in her career - could only convert one of seven break-point chances and racked up 37 unforced errors, winning just 29 per cent of points on her second serve. "I played some really tough players".
The losing semifinalists each earned $126,450 US.
"I'm disappointed with my singles".