The Japanese world number two went down 6-4 3-6 6-1 as an inspired Muchová of the Czech Republic earned her third win over a top five player this season.

"It was a tough match today, but we had some preparation," said Muchová.

"I was ready to play with all I had today, and to use all the weapons I could and [do] what I can.

"I'm definitely happy it worked out today."

Next up for Muchová will be Greek 16th seed Maria Sakkari who defeated Anett Kontaveit of Estonia 6-3 6-1.

Another former world number one to fall was sixth seed Karolína Plíšková of the Czech Republic who lost 6-0 7-5 to Russia’s Anastasia Pavlyuchenkova.

Pavlyuchenkova will face 11th seed Jennifer Brady of the United States after she dispatched former French Open champion Jelena Ostapenko of Latvia 6-2 6-1.

Two-time Madrid Open champion and third seed Simona Halep booked her place in the third round following a straight sets victory.

The Romanian won the first nine games before sealing a 6-0 6-4 triumph against China’s Zheng Saisai.

Twelfth seed Victoria Azarenka of Belarus was forced to withdraw from her second round clash with American Jessica Pegula with a lower back injury.

Pegula advances to face another Belarusian in fifth seed Aryna Sabalenka who eased to a 6-3 6-3 win over Daria Kasatkina.

The men’s singles competition also got underway today in the Spanish capital with Canadian Denis Shapovalov thrashing Serbian Dusan Lajovic 6-1 6-3 in the opening round of the Association of Tennis Professionals Masters 1000 event.

Shapovalov will now meet Kazakhstan's Alexander Bublik who overcame Hungarian Marton Fucsovics with a 7-6 (7-5) 6-3 success.

American Tommy Paul has set up a meeting with in-form Russian sixth seed Andrey Rublev after beating home favourite Pedro Martinez 6-4 7-5.

Australian Alex de Minaur also saw off Spanish opposition as he came from a set down to beat Jaume Munar 4-6 7-5 6-1.

Competition is due to continue tomorrow.