The American world number one thrashed Russian Ekaterina Makarova 6-1 6-3 in their semi-final on Friday, setting up a clash against Caroline Wozniacki in the final.
Despite having not dropped a set at the final grand slam of the year, Williams questioned whether she would have enough to progress so deep.
A win away from her 18th grand slam title, the 32-year-old said she was delighted with an appearance in the final.
"I'm just really excited to be in the final," Williams said.
"In the beginning of the week I definitely wasn't sure I would make it this long. Definitely wasn't sure I'd be here.
"So I'm just elated, to be honest, to have made it this far."
It marks Williams' best showing at a grand slam this year, having previously progressed no further than the fourth round in majors in 2014.
Williams said it was the result of hard work, having been written off after her third-round exit at Wimbledon.
"I worked really hard for Wimbledon. I worked really, really hard, and I was really disappointed and sad and shocked that I wasn't able to win," she said.
"I worked hours, more than I worked before. Maybe it's just paying off now."
Williams holds an 8-1 win-loss record over Wozniacki, but she has been pushed to three sets in the two victories this year.
"I definitely expect another close match. She really knows my game well and knows how to play," Williams said.
"She's so consistent. I think that's one of the things that makes her really tough.
"So I just have to be ready for that and, again, just stay calm and just be able to relax and be happy.
"The past six months I would never thought I'd be here. I think it's just staying calm and happy."