Halep won first, falling behind 3-1 in the first set but then reeling off seven games in a row to take total control against Carla Suárez Navarro, eventually finishing the No.3-seeded Spaniard off, 63 62.
"I was a little bit nervous at the beginning and she went up 3-1 in the first set, but from there I really tried my best and was just focusing on every point," Halep said after the match. "I had to be focused because she's a difficult player for me, and I'm very happy I was able to beat her in the end."
Halep is on an impressive run, winning 15 of her last 17 matches, including qualifying rounds.
"I played a lot of good matches with good players in Rome, and after that tournament I felt more and more confident," Halep said. "I've been feeling very good about everything, not just mentally, but also physically. I've been believing in my game a lot and playing more aggressively, and that's all."
Even more impressively, Halep won Nürnberg on clay last week, and is now in another final on grass.
"It's not easy even just to play on grass after playing so many matches on clay," Halep said. "So for me to be in the final this week, I'm really happy about it. I'm feeling good and playing well too."
It didn't take long for Flipkens to follow her into the final, as the No.4-seeded Belgian was never broken - she only faced one break point - and cruised past Spanish qualifier Garbiñe Muguruza, 63 61.
"The score was more easy than the match," Flipkens said. "In the first set there was only one break, and she's a really talented player and came out of qualies, so I knew it would be a tough match.
"But I was serving really well, and that was the difference in the end. And I had to be aggressive, because Garbiñe doesn't come off of the baseline, so I knew I couldn't just wait for her mistakes."
"She's on a roll," Flipkens said. "She won a tournament last week on clay and this week is in the finals on grass, so she's an all-around player, and she's in the best shape of her life. But I am too, so I'm really looking forward to playing the final. We'll see what happens. I'm really excited about it."
"It's going to be a tough match," Halep said. "It's going to be tricky because she slices the ball a lot, but I will try my best like I've been doing all week here, and we'll see how it goes tomorrow."
Flipkens has a better record in WTA finals than Halep, though - Flipkens is 1-0, Halep is 1-3.
The doubles final was also played, with No.2 seeds Irina-Camelia Begu and Anabel Medina Garrigues saving a match point down 9-8 in the match tie-break and beating Dominika Cibulkova and Arantxa Parra Santonja, 46 76(3) 119. Begu won her second WTA doubles title, Medina Garrigues her 20th.