When the heat was on, Bianca Andreescu found a way to raise her tennis game at the U.S. Open on a sweltering Wednesday night in New York City.

It was a hot and humid — stifling really — evening inside Arthur Ashe Stadium. Andreescu couldn't stop sweating. She wiped her face. She wiped down her arms and legs. She tried to stay focused.

But the early going of her quarter-final match against Belgium's Elise Mertens was proving to be nearly impossible for Andreescu to make a shot.

Down one set and frustrated throughout much of the night, the 19-year-old from Mississauga, Ont., seeded 15th, was nonetheless able to compose herself under the bright lights, battling back against the No. 25 seed Mertens to win a gruelling three-set match, 3-6, 6-2, 6-4. The match lasted two hours and one minute.

Andreescu is now the second Canadian to ever advance to the U.S. Open semifinals — after Carling Bassett in 1984 — and the fourth woman in history to reach the semifinals in her Open debut. Andreescu will play the No. 13-ranked Swiss tennis player Belinda Bencic in Thursday night's semifinal.

Andreescu has won 12 consecutive three-set matches and is 17-3 this year when the matches go the distance.

"This is honestly so crazy. A year ago I was in the qualifying round suffering from a back injury. I'm speechless," she said in her post-match on-court interview.

"I need someone to pinch me right now. Is this real life? Is this real life?"

Andreescu celebrates a point during her quarter-final victory over Mertens. (Elsa/Getty Images)

Needed to refocus

The New York crowd roared as Andreescu stood at centre court after her victory, amazed by what she had accomplished. It wasn't easy though.

Andreescu was feeling the pressure early in the match. Mertens was taking the game to the young Canadian, painting the lines with perfect form and forcing Andreescu into difficult situations on the court.

Andreescu had 14 unforced errors in the first set as Mertens cruised to an easy 6-2 win. Mertens had dropped just 16 games coming into the match, the fewest among the remaining players at the U.S. Open.

The game start had been delayed by 20 minutes due to the men's quarter-final. That meant the crowd had to exit the stadium and re-enter before the Andreescu-Mertens match. They took their time.

The fans also seemed disinterested at first, talking loudly and creating a distracting murmur. The players pushed on as people continued to talk loudly in the stands.

Andreescu needed to refocus for the second set if she wanted to stay alive at the U.S. Open.

Asked what she was thinking after dropping the first set, Andreescu didn't mince her words in the post-match interview.

"Get your [expletive] together," Andreescu said frankly. Cue the comeback.

Merten digs to return a shot to Andreescu. (Al Bello/Getty Images)

Power and prowess

The turning point came in the second set at 2-2. Andreescu found herself down 0-30 in her service game and needed a spark. That's when she lunged at a brilliant cross-court shot by Mertens and played a perfect volley.

That point changed the game. From there, Andreescu won the next 12 of 13 points.

From that point, she showed the power and prowess many had been accustomed to seeing from the young tennis phenom.

Re-energized, Andreescu started ripping forehand winners, bouncing around the court and pumping her fist after winners.

"Come on!" she yelled. "Let's go."

Andreescu forced a third and deciding set with a decisive 6-2 second-set win.

Andreescu serves to Mertens during their quarter-final match. (Elsa/Getty Images)

The two would trade games early in the third before Andreescu broke Mertens at 4-4. Andreescu served out the match in front of a capacity Arthur Ashe crowd.

"I've been working a lot on my fitness and mental strength," Andreescu said. "It's all the hard work I've put in."

Andreescu finished last season ranked 178th in the world. But with a record of 43-4 and two WTA wins already this year, she'll be in the top 10 rankings as the last major of the year comes to a close.

Prior to Wednesday night's match, Andreescu said it was a "dream come true" to be walking onto the Arthur Ashe Stadium court in prime time to be playing a quarter-final match-up.

But it's clear the teenage tennis star isn't content with a semifinal appearance. She wants this dream-like season to include two more wins at the U.S. Open.