The defending champion, who must keep winning to remain as world No.1, was expecting a stern examination against her Belgian rival, but the stiff challenge anticipated never materialised.
Mertens has been collecting silverware for fun this season and she arrived in Madrid on a 12-match winning streak. Her hopes of posing a legitimate threat to Halep were not helped by illness, though, and she was flat and error-prone throughout the opener.
The Belgian did perk up in the second set – performing with greater conviction and belief – but she was unable to deny her focused and determined rival a 14th consecutive victory in the Spanish capital.
Halep capitalised on a tentative start from her opponent and immediately nabbed a breakthrough. Mertens floated an attempted backhand lob beyond the baseline to make the worst possible beginning.
The Romanian was dialled in, but her opponent was messy from the baseline. The world No.16 unleashed a miserly two winners and committed a mammoth 14 unforced-errors in a 21-minute opener she will be keen to banish from her memory bank.
Matters got decidely worse for Mertens in the second set. She nudged a forehand wide to slip a break and 1-0 behind. It was in danger of becoming rout, but the Belgian eventually rallied.
Mertens got her side of the scoreboard moving after conceding the first 8 games. She cracked a wry smile after finally arriving to the party, and her relief was palpable.
Halep was forced to defend as her opponent began to find good depth with her groundstrokes as the set progressed. She rose to the challenge, though, and successfully fended off three break-points to maintain her slim advantage.
Stepping up to the line at 3-5, Mertens was attempting to prolong the contest, but despite a valiant effort, she was unable to prevent Halep from registering another victory.
It took the Romanian until her fifth match-point to seal the deal, and she will tackle Kristyna Pliskova in the third round tomorrow. The Czech qualifier overcame Sara Sorribes Tormo 7-5 6-2 to earn a shot at the best player in the world.
Another Czech, Petra Kvitova, is also through. The Prague Open champion continued her winning run by defeating Monica Puig 6-3 7-6(8).
Johanna Konta suffered an early exit after a second round loss to Bernarda Pera.The American, who defeated the Brit at the Australian Open in January, was a 6-4 6-3 victor in the Spanish capital.