The first set, which Andreescu won 7-6, lasted an hour and eight minutes - that single set was almost as long as the first semifinal of the evening.
It took some time for those tendencies to show themselves on Wednesday, but once they did Andreescu was her normal ferocious self.
Daniil Medvedev clinched his first appearance in the season-ending tournament with his victory over Stan Wawrinka in the U.S. Open quarterfinals.
This U.S. Open, though, hadn't even required that kind of heavy lifting.
"I just really like the challenge", she said of playing in big matches.
Monfils, the No. 13 seed, won the first set before Berrettini took the second. She converted her first match point by walloping a backhand down the line. Andreescu stormed back from 3-6 to win the next two sets convincingly, 6-2, 6-3, in just over two hours in total to advance to Thursday's semifinals. All of a sudden it was a 4-1 lead in the set for Bencic.
Andreescu uses meditation and visualization to keep her mind sharp and said the techniques she learned as a child help her deal with the pressure of the moment. Two games later, Andreescu gutted out three match points, finally converting on the third when Bencic couldn't handle a deep return.
No one can blame her.
Bencic is joined in the semi-finals by our outright Q2 pick, Bianca Andreescu, who gave us a nice quarter victor last night beating Elise Mertens in three sets, and being the first player to win a set from the Belgian in the tournament, in the process.
Her route to the final has included wins over former world number one Caroline Wozniacki and rising Belgian Elise Mertens.
The result puts another notch on Andreescu's remarkable streak of 22 straight wins in completed matches, a run that dates to late February.
Belinda Bencic powered her way into her first Grand Slam semifinal, wearing down Donna Vekic for a 7-6 (5), 6-3 victory at the U.S. Open. "She has a very good serve". A routine hold, and the kid from the Toronto suburbs would spend at least one more sleep in NY. "I didn't know there would be a pop quiz", she said.
Having started the year outside of top 150, she now finds herself on the cusp of an entry into the 10 - a meteoric rise led by her aggressive, hard-hitting style of play. She's just been taking it in, winning matches, doing her thing. "Is this real life?" she mouthed to them twice.
Andreescu faced Belinda Bencic in a semifinal match Thursday night. But she has won 32 of 36 matches this season, titles at Indian Wells and Toronto and climbed to No 15 in the rankings.
Nadal, who has won three of his 18 Major titles at Flushing Meadows, is playing in his 40th career Grand Slam quarter-final.
A year ago, Williams lost a controversial and chaotic final in NY to Naomi Osaka. She did not dispute the point. "Hopefully I can play my best tennis". But she's careful to remain just this side of cocky, too.