Bertens looked dominant throughout the summit showdown as she eased past reigning French Open champion Halep 6-4, 6-4 in nearly one and a half hour to lift the title.
Bertens was in control throughout the first set and demonstrated fighting spirit that should serve her well in the final to come from a break down in the second, winning five of the next six games to progress 6-2 7-5.
Osaka fell for the second time in two months to Federer's Swiss compatriot Belinda Bencic, exiting 3-6, 6-2, 7-5.
After a breakout 2018 season, which saw her win the biggest title of her career in Cincinnati - defeating four Top 10 players - Bertens qualified for her first WTA Finals and has continued to build on her success in 2019.
"It's a nice feeling to be back in the final of this special tournament". "I think I learned a lot from past year [a defeat to Petra Kvitova] - for sure, I will go for it".
"I am sorry to announce that I must withdraw from today's match". With my shots when I try to hit it a bit higher, and with more spin on that, she can not do that.
They have already played two matches on clay courts, and the balance is 1-1, with Halep who won the last match on this surface in 2017, in Rome.
World number three Halep can move back to the top of the WTA rankings with a win in Saturday's final. "But always when I step on the court, I'm ready for a three-hour match, so I'm OK". Bencic could not handle a vicious return to her feet, netted a volley-her first net error in 13-and Halep broke. "I played some good tennis and will be number four in the world on Monday, it is just all unbelievable things", Bertens added. "It was opposite today, I was mostly the defense, and Simona was the aggressive one, so I think that makes the difference as well". "But overall, I'm pretty happy how I fought". I wanted to do that, but I was thinking about it while I was playing, which wasn't a good thing. After a hold, she broke to clinch the set with another backhand victor to send the match to a third set. I am really proud of this week.
Halep had no complaints, congratulating her opponent while admitting she played into Bertens' hands.
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