Bencic had defeated Halep in their last two matches, including a Dubai quarter-final in February.
Kiki Bertens is back in the WTA Madrid Open final for the second straight year. "And she deserved to win the title because she played the whole week really good tennis". It's the second year in a row in the final for Bertens, who lost 7-6 (8-6), 4-6, 6-3 to Petra Kvitova in the 2018 final.
She also became the first woman to win the championship with dropping a set.
Third-seeded Halep found herself in an unusual semifinal against unseeded Belinda Bencic of Switzerland.
The Belgian saved eight match points to force a decisive tie-break, with Venus unable to shake off an opponent 15 years younger than her. She broke Bencic in the fifth game of the third set and then held through to the ninth game to serve for the match at 5-4.
The defeat was extra painful for Halep, though, who would have regained her number one ranking had she won the showpiece. "I think at the beginning, I was a little bit searching for my game still, but I think the rest of the match I played really solid, really smart, and I was just going for my shots", said Bertens.
The next game saw Bencic's backhand do the damage, setting up two break points and forcing an error off of the Osaka racket on her second chance. He lost in the last four in both Monte Carlo and Barcelona, arriving in Madrid without winning either tournament for the first time since 2015. She's the only player to do this in tournament history.
A more aggressive approach was paying dividends for Halep but it was high-risk too and when the Romanian blasted out in the very next game, Bertens was back up a break. Unhappy with her tennis and with life on the tour, Bertens spent the off-season taking stock on what she wanted out of her career.
It will come down to a battle between Bertens and her power game against Halep's more polished all-around play.
Roger Federer survived two match points against French maverick Gael Monfils to extend his claycourt comeback and reach the quarter-finals of the Madrid Open on Thursday.