The Rogers Cup win would be Andreescu's second professional win of her career, and she handled it with class.
When Williams explained her pain to Andreescu, she responded, "This sucks".
Williams, who is due to compete in Cincinnati next week, said she has experienced back spasms a lot in her career and that if this latest episode plays out like those before, she does not expect it to keep her out of any events.
Since returning to competition after Olympia's birth she has reached three Grand Slam finals - and she remained optimistic that she would be ready to try to match Australian Margaret Court's record of 24 Grand Slams when the US Open starts on August 26.
A relieved Andreescu, in her first tournament since she withdrew from the French Open ahead of the second round, dropped to her knees and kissed the blue hardcourt, while the home crowd chanted her name.
'I'm sorry I couldn't do it today.
Williams entered the match looking for a fourth Rogers Cup title. "I've dedicated so much hard work and sweat on that tennis court and in this gym, so this tournament is definitely ten times more special". My shoulder's good. I'm in the finals of the Rogers Cup. "(She has) a great serve - just really hungry". I was so, so nervous and I just felt like crying after, because I'm just so happy. "I tried but I just couldn't do it", an emotional Williams, who struggled to get the words out, told the crowd. Nadal will face eighth-seeded Daniil Medvedev, a 6-1, 7-6 (6) victor over sixth-seeded Karen Khachanov in the all-Russian first semifinal. I'm the first Canadian who got to the finals and has won this tournament since 1969, ' Andreescu said after being presented with the Rogers Cup trophy in an on-court ceremony.
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It didn't pan out perfectly given Williams' match retirement, but it was nevevertheless riveting stuff.
"This wasn't the way I expected to win", Andreescu said afterward.
"For sure there's some pressure, but I have nothing to lose, really", Andreescu said. I've had many, many tough matches'.
The 19-year-old Andreescu, from nearby Mississauga, Ontario, has victories over seven of the top 10 players in the world. "I kept telling myself 'never give up.' I'm trying to look at my injury not as a setback but more of a challenge".
Andreescu will rise to 14th in the world on Monday, replacing Amanda Anisimova as the highest-ranked teenager in the women's game. "I tried but I just couldn't do it".
Andreescu and Serena were both mic'd up, and you can hear the former ask if Serena was okay as she embraced her in a hug.