The 36-year-old American, seeded 13th, knocked off unseeded American CoCo Vandeweghe 6-7 (3), 6-2, 6-3 on Thursday to advance to the Australian Open final in Melbourne.
"Oh my gosh, (the win) means so much, mostly because she played so well," Williams said of Vandeweghe. "She played so unbelievably. I had to play defense the whole time. There was never a moment of relaxation the whole time. To be able to get to a final like this, I'm excited, and I'm excited about American tennis as well."
Williams will face either her sister, second-seeded Serena Williams, or unseeded Mirjana Lucic-Baroni of Croatia in Saturday's title match. The Williams sisters have met in eight Grand Slams finals, with Serena winning six times.
The most recent title showdown between the sisters also marked Venus Williams' last Grand Slam final before this week -- at Wimbledon in 2009, when Serena Williams posted a 7-6 (3), 6-2 victory.
Venus Williams' last major title came at Wimbledon in 2008, when she beat her sister 7-5, 6-4 in the final. Her last Australian Open final was in 2003, when she lost to Serena.
Williams and Vandeweghe broke serve once in the opening set, but Williams converted on her lone break point while Vandeweghe was 1 of 4 on her break opportunities. In the tiebreaker, Vandeweghe won four consecutive points to go up 6-2. Williams saved the first set point but committed a backhand error on the second.
Vandeweghe held serve to open the second set before Williams reeled off five games in a row. Williams finished the set with an ace.
Williams broke serve to open the third set. Vandeweghe had two break points in the next game, but Williams staved them off to open a 2-0 lead.
Vandeweghe saved three match points at 3-5, but an unforced forehead error ended the contest.
On the struggle to close out the win, Williams said, "(I was thinking), 'Oh my god, Venus, it's match point, you're supposed to win this.' I don't even know what happened on the last one. It's just, 'Oh my!'
"Everyone has their moment in the sun. Mine has gone on a long time. I'd like to keep it going. I have nothing else to do."
Vandeweghe, 25, advanced further than she ever had in 24 previous major events. Her best Grand Slam result before this fortnight was a trip to the 2015 Wimbledon quarterfinals, where she lost to Maria Sharapova.
Serena Williams was scheduled to meet Lucic-Baroni later Thursday, where the tournament was celebrating Australia Day.
Regarding the possibility of meeting her sister for the championship, Venus Williams said, "It's an unbelievable thing to watch Serena Williams play tennis. That girl can hit the ball, she is such a competitor. More than anything, I would like to see her across the net from me on Saturday."