Earlier in the day Heather Watson had been knocked out by Simona Halep but there were no suggestions Konta would follow her fellow Briton as she reached the last eight in Florida 6-2, 6-2
Konta raced into a 4-0 lead in the first set against the 32nd seed and took the first set 6-2 with few concerns.
Konta, the 24th seed who beat Russian qualifier Elena Vesnina on Sunday, immediately broke Niculescu in the first game of the second set before surviving two break points on her own serve to move 2-0 clear.
The 24-year-old’s dominance continued and she eased to a 5-0 lead by breaking Niculescu for a fifth time after making her third break point count.
Niculescu did rescue some pride by breaking Konta and holding to make it 5-2 before Konta finally finished her off.
Watson crashed out with a straight-sets defeat to Halep.
Romania’s world No 5 clinched victory in the last 16 in Florida 6-3, 6-4 despite suffering a scare early on from the British No 2.
Watson broke Halep in her first two service games and led 3-1 in the opening set before Halep rattled off five straight games to take the first set 6-3.
Watson, currently ranked 69th in the world and a wild card into the tournament, was then broken again in the fifth game of the second set before breaking back to make it 3-3.
But yet another break put Halep 4-3 up before the pair held serve to give the Romanian the chance to serve for the match.
The 2014 French Open finalist then wrapped up the win with a love service game to advance into the quarter-finals.
Serena Williams’ 20-match winning streak at the Key Biscayne event ended last night with a 6-7 (3), 6-1, 6-2 loss to Svetlana Kuznetsova in the fourth round.
Williams was bidding for her ninth title in the event and her fourth in a row.
But, after a gruelling first set that lasted nearly an hour, the No 1 seeded Williams faded on a sweltering afternoon.
Kuznetsova, the No 15 seed, won with defence, extending points until Williams would make a mistake. The Russian finished with only 18 unforced errors to 55 for Williams.
In addition, Williams’ serve was uncharacteristically unreliable. She hit 13 aces but also had nine double faults and was broken six times.
Williams still hasn’t won a tournament since earning her 69th title at Cincinnati in August.
This is the first time that she hasn’t reached the Key Biscayne quarter-finals since 2000, when she lost in the fourth round to Jennifer Capriati. Also knocked out was No 2-seeded Pole Agnieszka Radwanska, the 2012 champion, who lost to No. 19 Timea Bacsinszky 2-6, 6-4, 6-2.
The defeat was only the fourth in 21 matches this year for Radwanska.
Williams lives 90 minutes from Key Biscayne and has long considered the event her home tournament. But the stadium was two-thirds empty for her match, partly because of weather so uncomfortable that some spectators draped towels over their heads seeking refuge from the sweltering sun.
Williams played a flawless tiebreaker, hitting two aces and three other winners. But after she lost serve to fall behind 3-1 in the second set, her play became more flat-footed. At least twice she failed to pursue shots from Kuznetsova.
In the third set, Williams tried hitting balls as hard as she could, and some went for winners. But she double-faulted to fall behind 4-1, and Kuznetsova held twice to close out the victory.
There was no smiling by either player as they met at the net to shake hands.
Kuznetsova, 30, won the tournament 10 years ago and is into the quarter-finals for the first time since 2009.
She improved her record against Williams to to 3-8 and beat her for the first time since the 2009 French Open quarter-finals.