Williams stormed her way into the US Open final earlier in the day with a convincing win over Elina Svitolina.
For the American, it’s her 10th appearance in the final showdown at Flushing Meadows.
But the second women’s semi-final featured two women battling to reach their first career Grand Slam final.
The match was expected to be a tight contest from the outset and proved as much in the first set, with neither player being able to find a breakthrough before heading into a tiebreak.
Andreescu though found the extra gear to take the first set lead and put her in control.
Bencic however came out fighting in the second set, getting a quick double break to take a 4-1 lead.
But Andreescu soon recomposed herself to quickly break back twice to give her all the momentum in the second set.
The 19-year-old had the backing of the American crowd as she pushed Bencic before bringing up three match points on her opponent’s serve.
Eventually she got the breakthrough she needed to get the job done to reach her first career Grand Slam final in her maiden campaign in the US Open main draw.
The achievement clearly overwhelmed Andreescu as she immediately put her head in her hands in disbelief.
The Canadian incredibly wasn’t even born when Williams won her first US Open title back in in 1999.
And Andreescu called it a “dream” to face off against the 23-time Grand Slam champion in the women’s final on Saturday.
“It’s just surreal,” she said.
“It’s a dream come true to play against Serena in the final of the US Open.”