Niculescu was the first to advance to the final four, following up her upset over No.3 seed Klara Koukalova from earlier in the tournament with another surprise against No.8 seed Polona Hercog, and battling to the bitter end to do it - she finally won in two hours and 41 minutes, 6-2, 2-6, 7-6(3).
"It was a really tough match and I'm really happy about this victory," Niculescu said. "It started to rain a little bit so we went on the court later, but then I started the match well. She started to play well too, and so then we had to go to a deciding tie-breaker, and I was very happy to win the match there.
"It was so close - somebody had to win though, and I'm happy I was the one!"
The No.1-seeded Halep followed with a much simpler 6-3, 6-1 victory against Lara Arruabarrena, needing just 55 minutes - and just 21 minutes in the second set - to cruise through to the semifinals.
Halep beat Niculescu in the pair's only previous meeting, on the clay of Brussels in 2012.
Despite that, and despite Halep's meteoric rise over the last year, Niculescu's feeling good.
"I'm really happy two Romanians are in the semifinals - that means a Romanian will be in the final," Niculescu said. "Simona and I are good friends too, so hopefully we'll play a good match tomorrow.
"And I hope all the supporters will come out tomorrow too because it's Saturday!"
The other semifinal will pit No.2 seed Roberta Vinci against Kristina Kucova. Kucova was the first of the pair to move through, rallying past Danka Kovinic, 6-4, 1-6, 6-3, and Vinci follwed with a 7-5, 6-3 win against No.7 seed Petra Cetkovska, finally closing the match out on her fifth match point.
Vinci is now through to her first WTA semifinal of the year; Kucova is in the first of her career.
Vinci and Kucova will be playing against each other for the first time.