Yet Halep was able to quickly regain the initiative on this occasion and said: "I didn't think too much that I am again in the same position".
"I served pretty well, and the work that I did after the US Open, I could see it on court today".
She got her revenge here, though, taking advantage of Ostapenko's weak second serve with two breaks in the first set. It's the best moment in my life and I want just to keep it.
In the men's draw, world number one Rafael Nadal beat Grigor Dimitrov to set up a tasty final showdown with temperamental Australian Nick Kyrgios, who pulled off a stunning 6-3, 7-5 win over starlet Alexander Zverev. Still, she played really well those games, those next games [after falling 3-2 down]. Her win advanced her to the quarterfinals. "I wasn't as sharp".
"We all know what she's capable of doing, so I'm excited to go on court and to have another opportunity to improve and go for it". She did all the right things.More news: Top pick Garrett set to make NFL debut
Kasatkina hit 42 winners to Radwanska's 33 in a match that could have gone either way as both players racked up more than 40 unforced errors apiece.
On the other side, Jelena Ostapenko was playing her third consecutive semifinal in Asia trying to reach her first Premier Mandatory final in the process.
Halep's quarter-final victory on Friday over 34th-ranked Kasatkina avenged a straight-sets loss in their last outing last week at the Wuhan Open.
But the 20-year-old added after seeing off Romanian Sorana Cirstea 6-4, 6-4: "If I play my best and I stay aggressive tomorrow and I am consistent, I think everything can happen".