New York Liberty forward Breanna Stewart underwent an MRI and additional testing on Monday, sources told ESPN's Ramona Shelburne, to further examine the left knee injury suffered in Sunday's 76-69 win over the host Phoenix Mercury in Game 1 of their WNBA first-round series.
Stewart was injured in overtime with 3:01 left, when she attempted a layup against Phoenix's Satou Sabally and DeWanna Bonner. As Stewart went up, she already appeared to be grimacing, and as she landed on the court, she immediately grabbed at her left knee.
Stewart initially stayed in the game, but Liberty coach Sandy Brondello said that Stewart asked to be subbed out after feeling "discomfort." As Stewart sat on the bench, she looked visibly in pain and had tears in her eyes.
After the game, Brondello didn't provide an update on Stewart, who finished with 18 points. "We're just hoping that she will be OK," said Brondello.
Stewart missed 13 regular-season games because of a bone bruise on her right knee, and in March, she had a scope on her right knee.
Despite Stewart's exit, New York took Game 1 and stole home court from the Mercury.
Natasha Cloud, who played in Phoenix last season, made back-to-back layups to give the Liberty a two-point lead with 4:17 left in the fourth quarter. Kahleah Copper answered with two free throws, and after a turnover by Sabrina Ionescu, Alyssa Thomas made a no-look pass to Copper for a wide-open layup that gave the Mercury a 65-63 lead with 1:44 remaining.
Stewart then made a tough basket in the lane 13 seconds later that eventually forced overtime.
"We looked around and realized we kind of have been in that position before," Ionescu said. "Understanding how important overtime is and knowing we had another gear to get to. I'm proud of the way we stuck together; we weathered the runs they went on and ultimately found a way to win."
Cloud hit a 3-pointer to open overtime and spark an 8-0 run that made it 73-65 with 1:51 left. Phoenix trimmed its deficit to four, but Leonie Fiebich, who finished with 10 points, hit a huge 3-pointer to seal the win.
"She had missed some early on, but I knew she was going to hit the biggest one of the night," said Ionescu about Fiebich's shot. "It's what she does ... she's one of the best 3-point shooters in the world."
The Liberty, who averaged 13.8 turnovers in the regular season, finished with 20 turnovers -- one shy of their season high -- which included 14 after halftime.
With or without Stewart, the Liberty can sweep the series in Game 2 on Wednesday and advance to the semifinals for the second season in a row.
Despite Stewart's uncertain availability, the Liberty do not want to squander a chance to close out this series quickly.
"We have the advantage," said Brondello. "We won on their home court, and now the advantage is for us to take care of it. This is an experienced team, and we've been here before. We know what our goal is, and we want to keep playing."
The Associated Press contributed to this report.