Cirstea had lost four of her previous five meetings against Li but came out firing from the beginning, winning a 6-1 first set in just 22 minutes - Li managed only 12 points in that lopsided opening set.
The second set was the complete opposite, with Li storming out to a 5-2 lead and seemingly about to push the match to a third. Cirstea reeled off four games in a row from there to go up 6-5 and built a 40-15 lead, double match point - she missed on both of them, one with a forehand down the line that landed just wide and the other on a backhand into the net - Li then pushed the match to a breaker.
And the No.4-seeded Li threatened to take the set again, racing out to a 4-1 lead, but Cirstea came back one last time, winning six of the last seven points to finally close the match out, 61 76(5).
"I think I came on the court with a really good mentality today," Cirstea said afterwards. "I was very focused from the start, and even when she started to push me in the second set, I was still playing quite well, except those two match points at 6-5, of course, when I think I got a little bit tight.
"I'm happy with how I fought in the end and I'm really happy to finish the match in two sets."
Cirstea is now through to the third WTA final of her career, her first two coming at much smaller International-level events at Budapest in 2007 (where she was runner-up to Gisela Dulko) and Tashkent in 2008 (where she won the title, saving a match point to beat Sabine Lisicki in the final).
"I'm not thinking so much of what was in the past now - I'm just thinking of what's coming," Cirstea said. "Every single match this week is a new match. It doesn't matter who I beat the day before or what I've done. It's another day where I have to get out there and show the work that I've been putting in.
"I'm really excited, because these are the moments you're working for."
And it's not like Cirstea has been MIA since that last WTA final five years ago - she made 10 WTA semifinals in that span, including Premier semifinals at Los Angeles in 2009 (falling to Sam Stosur), Stanford in 2012 (falling to Serena Williams) and Stanford this year (falling to Dominika Cibulkova).
"I thought I could win the second set, but when I was up 5-2 in the set and 4-1 in the tie-break, I just didn't catch my chance," Li said. "She played a little bit differently today than she usually plays. Before she was always shoot, shoot - but today she was playing topspin sometimes. Sometimes I was overplaying my shots too, and that's why I made a lot of mistakes and gave her a lot of free points.
"But it wasn't bad. At least I was in the semis. Of course when you lose a match you're always a little bit sad, but you still have to find the positives. It was my first tournament after Wimbledon and I have a lot of things to think about going forward. I did a pretty good job; I'll just continue from here."