Mertens fell to a 7-6, 7-6 defeat at the hands of her unranked compatriot, whose reward is a meeting with Estonia's Anett Kontaveit.

Kontaveit comfortably overcame Turkey’s Cagla Buyukakcay 6-1, 6-2.

Also suffering a shock exit was number three seed Nao Hibino after the Japanese lost 7-5, 6-3 to unranked Francoise Abanda of Canada.

The lone seed in the top three to win was number two Magda Linette of Poland, a 7-5, 2-6, 6-3 victor at the expense of the United States' Asia Muhammad.

Among the other notable winners in the women’s competition was 15-year-old Russian wildcard Anastasia Potapova, who defeated fifth-ranked Maria Sakkari of Greece 6-4, 0-6, 7-6.

World number 84 Sakkari is ranked 370 places above Potapova in the Women's Tennis Association (WTA) standings.

Japan’s Risa Ozaki, the 10th seed, proved too strong for Great Britain’s Katie Swan in a 6-3, 6-3 triumph.

The men's competition also yielded a surprise result as Germany’s Tobias Kamke came from behind to defeat number two seed Pierre-Hugues Herbert of France 3-6, 6-3, 7-5.

The shortest match of the day saw 15th-ranked Dusan Lajovic of Serbia defeat Slovenia's Grega Zemlja by means of retirement after 32 minutes.

Lajovic, who reached the round of 16 at last week's Indian Wells Masters, had won the first set 6-3 prior to Zemlja’s withdrawal.

Also winning by means of retirement was Moldovan sixth seed Radu Albot, who was winning 6-2, 5-1 when Swedish opponent Elias Ymer pulled out. 

Qualifying rounds are due to continue tomorrow to determine the 12 additional slots in each singles draw.

The main draw is scheduled to begin on Tuesday (March 21) as the women take to the court.

It will be followed by the first round of the men's main draw on Wednesday (March 22).