Williams earned her 10th trip to the final at the Flushing Meadows with a 6-3, 6-1 victory in one hour and 10 minutes over No. 5 Elina Svitolina Thursday night at the Arthur Ashe Stadium.
The match was, by no means, as easy as the American's 44-minute quarterfinals win over Qiang Wang.
By the time this match hit the 44-minute mark, the first set had just ended. The first two games alone lasted 16 minutes.
Svitolina had her chances throughout the first set, but Williams fought off six break points.
Once the 37-year-old closed out the first set, she made quick work of the second to win her 101st career match victory at the US Open, tying Chris Evert's all-time record.
"They were long," Williams was quoted as saying by the official website of the US Open about the first two games in the first set. "She's such a good player, obviously. Two semis in a row is really impressive and really hard to do. I just wanted to not get off to a slow start and just try to hang in there."
In the other semi-final, 19-year-old Andreescu defeated No. 13 seed Belinda Bencic of Switzerland 7-6 (7-3), 7-5 in a match that lasted two hours and 12 minutes to reach her first Grand Slam final.
Andreescu never held a break point in the first set, but won it in a tiebreak after saving all six break points against her, then fought back from a double-break down in the second set to reel off the last five games of the clash and claim victory to set up summit-clash date with Williams which will take place on Saturday afternoon.
"It's the hard work that I've put in that has materialised through this year. It all started in Auckland for me but I think it was the Indian Wells where I felt that I could come through the big matches," Andreescu said in the on-court interview after the match.
"Serena has been an inspiration for me and I can't wait to get back on the court and play against her," she added.
In their only prior meeting, Williams and Andreescu faced off in the Rogers Cup final earlier last month, but the match ended abruptly after the American legend was forced to retire due to an upper back injury.
( With inputs from IANS )