Naomi Osaka of Japan recorded a major upset as she sent defending champion and No. 6 seed Angelique Kerber of Germany packing with a 6-3, 6-1 win in the first round of the US Open at Arthur Ashe Stadium. The 19-year-old Osaka hit 22 winners and committed 17 unforced errors on her way to her first win against a Top 10 player in just 65 minutes. “I just really want to play good here," said the World No. 45 Osaka after the match. “I played good today and so I hope to carry that on to my next match.” Osaka’s win was the biggest for a Japanese player in a major tournament since Akiko Morigami stunned then World No.3 Nadia Petrova at Roland Garros in 2006.

Kerber struggled with her serves, committing her fourth and fifth double faults in the eighth game of the first set that allowed Osaka to clinch the first break of the match. The next game, Kerber clinched two break points but Osaka hit a backhand winner and a forehand down the line to save both. Two points later, the German netted a return that gave Osaka the first set. In the second, Osaka broke Kerber twice on her way to her first Top 10 win in 10 chances. She will face the winner between Rebecca Peterson and Denisa Allertova.

Kerber was the second big name who bit the dust in the first two days of the tournament. Former world No. 1 Maria Sharapova, in her first major tournament since returning from a 15-month suspension, defeated No. 2 seed Simona Halep in straight sets.

The loss spoiled Halep’s attempt to clinch the No. 1 spot for the fifth time this WTA season. Last week, she was on course to clinch the top spot in Cincinnati, but she lost in the finals.

No. 1 Pliskova hurdled first-round rival

World No.1 and top seed Karolina Pliskova hit eight aces and 29 winners en route to an impressive 6-2, 6-1 win over world No.

72 Magda Linette in her opening-round match. It was a rocky start for Pliskova in her first match as the No. 1 seed in a major tournament, committing three double faults early in her clash with Linette. However, she regained her bearings as she finished the first set in just 38 minutes. "I think serve could be better, but overall, I think it was solid," said Pliskova, who will next face either against Veronica Cepede Royg or Nicole Gibbs in the second round.

Strycova, Cirstea cruised to second round

In other matches on the women’s group, No. 23 seed Barbora Strycova defeated Misaki Doi of Japan, 6-1, 6-3, to reach the second round. Romania's Sorana Cirstea followed suit with a 6-1, 6-3 win over Lesley Kerkhove of the Netherlands.