Bouchard, a 22-year-old former Wimbledon finalist, was beaten in the first round of the Brisbane International last week in her first warmup tournament for the Australian Open, which begins on Jan. 16 and where she was a surprise semifinalist in 2014.
The Canadian went on to make the semis at the French Open that year before losing to Petra Kvitova in the Wimbledon final. Bouchard finished that year ranked seventh, but injuries and poor form since have seen her ranking slide to 46.
Earlier at Sydney, two-time Grand Slam singles champion Svetlana Kuznetsova defeated Irina-Camelia Begu of Romania 6-1, 6-4. Defending champion Kuznetsova broke the Romanian's serve in the ninth game of the second set, then held serve to advance to the second round of the ATP-WTA tournament.
Yulia Putintseva of Kazakhstan advanced when Belinda Bencic of Switzerland, who partnered Roger Federer in last week's Hopman Cup, retired with a toe injury when Putinseva was up 6-4, 2-3.
In a first-round men's match at Sydney, France's Nicolas Mahut beat Thomaz Bellucci of Brazil 6-2, 7-6 (2).
Pliskova, who beat Cornet 6-0, 6-3 at Brisbane on Saturday evening, withdrew from the Sydney International with a left inner thigh injury. Pliskova, who would have been seeded fourth in Sydney, was replaced by lucky loser Roberta Vinci of Italy.
Sydney officials also announced seventh-seed Elina Svitolina of Ukraine withdrew due to a viral illness.
"I've had constant headaches for the last two days and unfortunately in today's hot weather it got even worse when I was warming up and my body wasn't ready to compete," Svitolina said.
Svitolina was scheduled to play Olympic champion Monica Puig on Sunday, but Puig will now play a lucky loser from qualifying on Monday.
No. 1 Angelique Kerber and third-ranked Agnieszka Radwanska remain in the Sydney draw.
At Hobart, defending champion Cornet withdrew from the tournament with a back injury, a Tennis Australia official said.
American Shelby Rogers posted the first upset of that tournament in the opening match, beating second-seeded Anastasija Sevastova of Latvia 6-2, 1-6, 6-4. Lizette Cabrera of Australia continued the Hobart trend, beating fifth-seeded Misaki Doi of Japan 6-4, 5-7, 7-5.
Third-seeded Monica Niculescu defeated former No. 1 Jelena Jankovic 6-2, 6-3.
In a men's tournament at Auckland, New Zealand, top-seeded Roberto Bautista Agut and second-seeded John Isner headline the entries. Bautista Agut, Isner, third-seeded David Ferrer and No. 4-seeded Jack Sock have first-round byes in the tournament which begins Monday.