And her pep talk worked. Her win-loss tally, following her run in NY, now stands at 43-4 - numbers which were once matched only by the likes of Serena Williams and other prominent giants. She was up two breaks and then she handed one back as she double-faulted twice.
"Is this real life?" she screamed to her box - that also had her dog Coco - after winning her quarter-final match.
Ranked 178th at the end of the 2018 WTA (Women's Tennis Association) season Andreescu has rocketed up the world rankings and will be at least No. 6 when the final major of the year draws to a close this weekend.
She is the first player to get this far in her debut in NY since Venus Williams was the runner-up in 1997. It marked her 11th consecutive victory. This was the power game for which she had quickly become known. And now what I've accomplished this year, I'm honestly speechless. "That's maybe something she does very well".
Andreescu will next face fellow first-time semi-finalist Swiss Belinda Bencic.
Basketball Hall of Famer Steve Nash tweeted about her.
Their duel proved a more prolonged and engrossing one as two players with hugely impressive variety to their games proved extremely well matched.
Calling 2019 a breakthrough year for the 19-year-old Canadian will be an understatement. Such was her winning strategy against unseeded American qualifier Taylor Townsend on Monday night, during her fourth-round victory, and it's something she was able to do at times against the No. 25 seed Mertens. As a wild card at Indian Wells - her first premier tournament - she stunned the field, fans and the media alike with a dominant performance, which included wins over Garbine Muguruza, Elina Svitolina and Angelique Kerber, to take home the title. But with her solid battling wins, she has the chance to repeat the feat that world No 1 Naomi Osaka achieved previous year.
Andreescu will now face Belinda Bencic, who wore down Donna Vekic and advanced to her first Grand Slam semifinal with a 7-6 (5), 6-3 victory. "It's definitely not going to be an easy match", said Andreescu.
It took some time for those tendencies to show themselves on Wednesday, but once they did Andreescu was her normal ferocious self. "I think that's the whole tennis difference". Really, I'm taking it step by step. This sucks - on your back, right?
But then another writer noted that while it's true that she might not have imagined such a reality a year ago, it didn't seem that hard to imagine a couple of weeks ago, as she was ripping through her rookie season.
The 19-year-old Canadian became the youngest semifinalist at Flushing Meadows since Caroline Wozniacki was also 19 when she was runner-up in 2009.
Born in Canada to parents who had emigrated from Romania in 1995, Andreescu moved with her parents to their native country when she was 7.
A backhand victor from the Swiss brought up the first of two break points in the fourth game, but Andreescu survived both of them before staving off three in the sixth game.