Defending champion Simona Halep sauntered into the third round of the Madrid Open with a straight-sets demolition of the previously in-form Elise Mertens.

Mertens had won every match on clay this season ahead of her showdown with the world number one, claiming titles in Lugano and Rabat.

The Belgian has been struggling with illness, though, and her 13 match-winning run was ruthlessly halted by Halep, the world number one easing to a 6-0 6-3 victory on Tuesday.

Halep has made an ominous start to her bid to claim the title for a third win in a row at Caja Magica and prevent Caroline Wozniacki from replacing her at the top of the rankings.

The Romanian thrashed Ekaterina Makarova 6-1 6-0 in the first round and made light work of dispatching Mertens, taking only 71 minutes to extend her winning run in Madrid to 14 matches and set up a meeting with Kristyna Pliskova.

Halep lost only three points on serve in the opening set after Mertens double-faulted in the opening game to go a break down.

Mertens looked tentative and Halep made her pay, too many unforced errors costing the 22-year-old as the top seed defended brilliantly and had plenty of joy with her potent backhand down the line.

Halep wrapped up the first set in a flash and rallied from 0-40 down to break in the opening game of the second set.

There was a smile from Mertens after she held to get on the board at 2-1 and Halep saved a first break point to stretch her advantage to 4-2.

Mertens showed fight to save four match points, but another double fault sealed the excellent Halep's place in the last 16.