The world number six made headlines last month when she stunned Serena Williams in NY to win her maiden Grand Slam, in a final overshadowed by the American's bitter row with the chair umpire.
Osaka, 20, required treatment on her lower back midway through the second set. In the first last week, she got to the Pan Pacific Open final in Tokyo. Asked at the post-match press conference, where she again looked unhappy, why she had had tears in her eyes on court, she replied curtly, "Because it was a hard match".
"But my back's sort of been bothering me since the beginning of the tournament".
"I feel like we both have really good serves", she added.
Sevastova is set to play the final against the victor of the match between Caroline Wozniacki of Denmark, the second seed and victor of the Australian Open this year, and wildcard Wang Qiang of China.
Sevastova claimed one of the biggest wins of her career as she upset eighth seed Naomi Osaka 6-4, 6-4 to reach her first ever Premier Mandatory final.
"She didn't give me any presents, it was tough", said Sevastova, who retired in May 2013 because of persistent injuries, only to return in January 2015. In the men's quarter-finals, the Italian Fabio Fognini continued his march towards a fourth title of the season with a 6-4, 6-4 victory over Marton Fucsovics of Hungary.