Begu and Niculescu won a tournament together in Thailand last month
Romania reached their first Fed Cup World Group semi-final with a thrilling 3-2 win over defending champions and hosts the Czech Republic in Ostrava.
With the tie level at 1-1 after day one, Simona Halep got the better of fellow former world number one Karolina Pliskova to put Romania 2-1 up.
Katerina Siniakova then beat Mihaela Buzarnescu to set up the decider.
And Irina-Camelia Begu and Monica Niculescu defeated Siniakova and Barbora Krejcikova to seal victory.
The Romanian pair recovered from losing the opening set to win 6-7 (2-7) 6-4 6-4 in a match that lasted just under three hours.
French Open champion Halep, who has vowed to win a first Fed Cup for her nation before she retires, had dug deep to defeat Pliskova 6-4 5-7 6-4 before Siniakova ran out a comfortable 6-4 6-2 winner over Buzarnescu.
Siniakova and Krejcikova, the Wimbledon champions and the top two players in the doubles rankings, were looking to give the Czech Republic a 12th consecutive home win in the competition, which they have won six times since 2011.
They looked on track when they took the opening set but Begu and Niculescu broke early in the second set and went on to level the match.
A nervy third set saw three breaks of serve in a row at the start leaving the Romanian pair 3-1 up.
Siniakova and Krejcikova broke back to make it 4-4 and the momentum seemed to be with them before Begu and Niculescu held their nerve for a famous win.
Also through to the semi-finals are France and Belarus.
France led 2-0 after Saturday's play and Caroline Garcia, back in the French squad for the first time in two years after a falling-out with team-mates, sealed the win thanks to a 6-2 6-3 success over Elise Mertens, before Belgium got a point on the board thanks to a win in the doubles for Ysaline Bonaventure and Kirsten Flipkens, who beat Pauline Parmentier and Fiona Ferro 10-6 in a third-set tie-break.
Aryna Sabalenka and Aliaksandra Sasnovich guided Belarus to a 4-0 victory over Germany. The pair both won on day one while Sabalenka made sure of a semi-final spot with a comfortable 6-1 6-1 success over Laura Siegemund.
Victoria Azarenka and Lidziya Marozava completed the rout by defeating Mona Barthel and Anna-Lena Groenefeld in their doubles match.
