In a battle of former Rogers Cup champions, a ruthless Wozniacki showed no mercy as she stormed past her good friend and 10th-seeded Pole Agnieszka Radwanska 6-3, 6-1 in 78 minutes.
After taking the first set, Czech top seed Pliskova was given a scare after Osaka fought back to level in the second set tie-break.
Pliskova advances to face Japanese teenager Naomi Osaka, who posted a 6-3, 6-4 victory over 15th-seeded Anastasija Sevastova of Latvia.
Garcia ended the run of 18-year-old American CiCi Bellis with a 6-4, 6-2 victory over the American.
Pliskova meanwhile was left satisfied after pounding down 12 aces and not dropping her serve once.
She admitted that remaining No. 1 will be a challenge.
"I think the biggest mistake was in the first set - so because of that, I think I lost", Pliskova said. "I once I kind of got on a roll, I started serving better".
Williams, who was the oldest player in the draw, fell behind 5-0 in the first set and failed to muster any sort of challenge against fifth seeded Svitolina.
"Almost every tournament I've played this year there has been a rain delay, so at this point I just have to laugh because I feel like it is following me", said Wozniacki. "So I just think about what's in my control, and I was ready when we had to be ready".
The two players have not yielded an inch in the second set, which was decided in the tiebreaker.
"Last week I was just hoping to win a match at some point and beat someone".
"I think I was maybe a little bit less exhausted towards the end", Wozniacki said.
"I had a little bit [of nerves] before the match, but during the match I felt much better especially at the end", Pliskova said. "I was just trying to stay in the game, the first game of the third set was very tough I didn't even know she was injured".
In only her third tournament of the year, the American world number 934 continued her impressive run in Toronto, after victories over Angelique Kerber and Petra Kvitova.
Halep, the defending champion, bundled out Barbora Strycova of the Czech Republic in under an hour, winning 6-1, 6-0.