She eased into the last eight in Rome with a 6-3, 6-3 win over Mihaela Buzarnescu.

Osaka beat the 29th-ranked Romanian in 78 minutes after earlier beating Slovak Dominika Cibulkova by the same scoreline in one hour and 42 minutes in the second round, as players doubled up after the previous day was washed out.

At the start of the day, the 21-year-old had needed to make the quarter-finals to ensure Romanian Simona Halep could not swipe the number one ranking and with it top seeding in the French Open.

But Osaka’s top spot was guaranteed when reigning Roland Garros champion Halep fell to Czech Marketa Vondrousova 2-6, 7-5, 6-3 in the second round.

Osaka next meets Madrid champion and sixth seed Kiki Bertens for a place in the final four.

Osaka, who broke through by winning the US Open last year and Australian Open this year, did not survive the second round in her two previous appearances in Rome.

Before this year Osaka had never made the last eight of any clay tournament, but has also reached the semi-finals in Stuttgart and quarter-finals in Madrid as she warms up for the French Open start on May 26.