The match-up was always going to be a tricky one - Niculescu had beaten Li in their only prior meeting after all - but the No.1-seeded Chinese was just tough enough, using her powerful serve and ground game to hit through the Romanian's barrage of slices and short balls, eventually prevailing, 75 46 64.
"Monica plays with a very unique style for women's tennis," Li said. "When I play against her I need to be very careful and just try to keep playing my game, which can be easily disrupted by her. You have to work hard to find your rhythm against her and I did work hard, and I'm very happy to win this match."
Ahead of one of Li's favorite Grand Slams, the Australian Open, this kind of challenge was valuable.
"I need to play these kinds of matches before the Australian Open," the World No.3 added. "This way I can make all of the necessary adjustments I need to make, and I can increase my belief in myself."
Awaiting Li in the semifinals is No.8 seed Annika Beck, who beat Patricia Mayr-Achleitner, 36 63 61. From 36 33 Beck ran away with it, winning eight games in a row - and eventually nine of 10 - to end it.
Li and Beck will be playing against each other for the first time.
Beck will not only be going for her first Top 10 win against the No.3-ranked Li, she will be going for her first Top 20 win (the German teenager is 0-4 against Top 10 players and 0-6 against Top 20 players).
The other semifinal will pit No.5 seed Peng against Vania King, after Peng brushed aside Jana Cepelova, 63 62, and King beat Barbora Zahlavova Strycova, 62 64. King had just pulled off the upset of the week the day before, beating No.2 seed Sara Errani - she saved two match points to do it.
"I'm really excited," King commented after the match. "Barbora didn't play her best tennis out there today, but she's a great player and I'm looking forward to playing her many more times in the future.
"I didn't expect to make the semifinals here, but it's a pleasant surprise. My hard work is paying off."
Peng beat King in their only previous meeting, though it was almost four years ago.