CARTAGENA, Spain - Appropriately for a year in which radical changes to the Fed Cup format have shaken up the competition, the inaugural Qualifier rounds also began with upheaval on court as Sara Sorribes Tormo shocked Naomi Osaka 6-0, 6-3 in just one hour and 17 minutes, spearheading a 2-0 lead for hosts Spain over Japan after the first day of play.

The World No.78, who had taken Osaka to three sets in Madrid last year, delivered sublime defensive work to post her first Top 10 win over an opponent who never fully settled into the match. Using the slow clay to her full advantage, Sorribes Tormo was a brick wall blunting Osaka's power, committing just four unforced errors while cleverly varying the length and spin of her shots. An unsettled Osaka, playing the first away Fed Cup tie of her career, quickly lost the first six points of the match - and the opening set didn't get much better for the former World No.1, who would win just one point on serve during it.

Forcing herself forwards paid dividends for Osaka as she attempted to essay a turnaround: the Japanese No.1's best points of the day were all finished with drive volleys. Briefly, it seemed as though she might succeed when a fourth double fault from Sorribes Tormo - the Spaniard's only weakness today - put Osaka ahead 2-0 in the second set. But despite the marginal improvement, Osaka would fail to get her unforced error count under control, tallying an astonishing 50 over the course of the contest - 24 in the first set to just one winner, and 26 in the second - and coughing up a third double fault of her own down match point.

The upset paved the way to a commanding lead for the hosts. Japanese No.2 Misaki Doi came out firing with her lefty forehand at the start of both sets against Carla Suárez Navarro, but fine movement and a series of exquisite backhand angles enabled the former World No.6 to overturn a break deficit in the first and a double break in the second to emerge a 6-3, 6-4 winner, putting Spain up 2-0 at close of play. Japan now need to win all three remaining rubbers to progress to April's Fed Cup Finals in Budapest.

Elsewhere, Switzerland also took a 2-0 lead at home in Biel/Bienne over an injury-struck Canada. US Open champion Bianca Andreescu and former Wimbledon finalist Eugenie Bouchard were both on the guests' team, but neither were able to compete: Andreescu, still recovering from the knee problem that forced her to withdraw from the WTA Finals in October, was not named to the starting line-up, while Bouchard withdrew hours ahead of her clash with Belinda Bencic due to a wrist injury.

Instead, Canada were led by fast-rising 17-year-old Roland Garros girls' champion Leylah Fernandez, while two-time Grand Slam mixed doubles champion Gabriela Dabrowski took Bouchard's place against Bencic. The substitutions were not enough: World No.185 Fernandez was able to keep things close against Swiss No.2 Jil Teichmann, but the Prague and Palermo champion was able to stay just in front to ride out a 7-6(4), 6-4 victory. US Open semifinalist Bencic proceeded to stamp her authority as a freshly-minted Top 5 player on the tie, dominating throughout a 6-1, 6-2 dismissal of Dabrowski during which she struck six aces and won 81% of her first serve points.

One of the most intriguing Qualifier ties of the weekend is all square at 1-1 after both Dutch No.1 Kiki Bertens and Belarusian No.1 Aryna Sabalenka posted opening wins in The Hague. Bertens had to take the long way to victory against former World No.30 Aliaksandra Sasnovich: though the 25-year-old's ranking has sunk to World No.113, she has a history of playing her best tennis in Fed Cup action. A top-quality first set saw Sasnovich's smooth hitting and a series of fine dropshots hold off a Bertens comeback to edge the tiebreak.

However, buoyed by her home crowd, Bertens began using her preferred clay surface to her advantage, doggedly retrieving from behind the baseline while beginning to bring her formidable serve to bear. Racking up seven aces and many more service winners, the Madrid champion wrested control of the match as Sasnovich started to miss the mark; frustrated by her opponent's defence, the Belarusian netted consecutive putaways to drop serve in the fourth game of the deciding set, and never recovered from there as Bertens made her way to a 6-7(5), 6-2, 6-1 victory.

The guests' disappointment was short-lived, though: Aryna Sabalenka wasted no time in pulling one back for Belarus, striking 32 winners to dismiss World No.77 Arantxa Rus 6-2, 6-3 in one hour and 14 minutes - setting the stage for a tantalizing second day of the tie, with all eyes on the potentially crucial heavyweight clash between Top 20 players Bertens and Sabalenka.

Meanwhile, Romania and Russia also finished the first day tied at 1-1. Shenzhen champion Ekaterina Alexandrova, leading her team for the first time, made a stellar debut to dispatch Elena-Gabriela Ruse 6-1, 6-4 - but World No.90 Ana Bogdan delivered an upset to level the tie for the hosts in Cluj-Napoca, shaking off the loss of a tight second set to come through an emotional encounter 6-3, 6-7(5), 6-1 over Veronika Kudermetova.