Bianca Andreescu staying 'optimistic' for US Open after 'small stress fracture' in her back

WATCH: Bianca Andreescu lost to Camila Giorgi in the opening round in Montreal earlier this week.

2019 US Open champion Bianca Andreescu is staying "optimistic" in regards to competing at the year's final Grand Slam in light of a new injury setback: a stress fracture in her back.

The Canadian revealed her diagnosis in a social media post Saturday, announcing her withdrawal from the upcoming Western & Southern Open along with it. Andreescu said she started feeling pain in her back two weeks ago at the Mubadala Citi DC Open, where she failed to convert match points in a first-round loss to Marta Kostyuk, and "did everything [she] could" to compete at this week's Omnium Banque Nationale "at the highest level."

Andreescu triumphed at the WTA 1000 event in Canada weeks prior to her first major win four years ago, making her the first Canadian to win at home in 50 years.

"My main focus will be on giving my back the rest it needs," Andreescu added. "I'll be taking things day by day and aiming to return to the court as soon as I can and stay optimistic for US Open. Your support means the world to me.”

After losing to Giorgi in Montreal, Andreescu was asked by a reporter about the status of her health, as she met with the WTA physio—but didn't receive a medical timeout—in her 6-3, 6-2 defeat. In her answer, she revealed that she'd been struggling with pain in her SI joint, which links the pelvis and lower spine.

"I've had this before. I've had to deal with it many times, actually," she said. "It got better for a few days, and then I started playing again, and then obviously I felt some irritation, but I saw some light at the end of the tunnel.

"But with SI joint pain, it's very difficult because it's nothing very serious, but if you do push, you do feel a lot of pain. That's what I experienced one of the days practicing before the tournament."

At the time, Andreescu said she "didn't know" if she was going to play Cincinnati, and that "the goal" was being healthy enough to compete in New York.