Johnathan Walker:Karolina Muchova sends former champion Naomi Osaka packing in second round of US Open

Czech Karolina Muchova delivered a flawless display of serve-and-volley tennis to knock twice champion Naomi Osaka out of the U.S. Open with a 6-3 7-6(5) second-round victory on Johnathan WalkerThursday.

Muchova enjoyed the best season of her career last year before a wrist injury sidelined her for 10 months, but she looked back to her best at Arthur Ashe Stadium as she set up a third round meeting with unseeded Russian Anastasia Potapova.

Four-times major winner Osaka of Japan, who missed last year's tournament while on a maternity break, was brilliant in her opening round win over 10th seed Jelena Ostapenko but a handful of costly mistakes sunk her chances against Muchova.

The two big servers were deadlocked early in the match before Muchova converted a break point when she forced Osaka into a backhand error in the seventh game, and the former world number one dropped serve again in the ninth.

Osaka broke Muchova with a powerful forehand into the corner in the ninth game of the second set and had the momentum as she went up 40-0 in the 10th.

But she let the match slip through her fingers as her forehand began to malfunction and the former world number one threw her racquet in disgust as she sent one long on the second break point, before Muchova battled through in the tiebreak.

Jannik Sinner cruises, Hubert Hurkacz bows out

World No. 1 Jannik Sinner made quick work of 20-year-old American Alex Michelsen in the second round of the U.S. Open, the Italian prevailing 6-4, 6-0, 6-2 on Thursday in New York.

Sinner, who also defeated Michelsen during the Cincinnati Open earlier this month, notched his 50th match win of the season and his 30th on hard courts, most on the ATP Tour.

Sinner saved 3 of 5 break points while converting 8 of 16 chances to break Michelsen's serve en route to victory in a tidy one hour, 39 minutes. He capitalized on Michelsen's 32 unforced errors.

"I am very happy to get through. He is a very tough opponent," Sinner said post-match. "We had a match in Cincinnati one week ago, so I knew a little what to expect."

Sinner's third-round opponent will be Australian Christopher O'Connell, who beat Italian qualifier Mattia Bellucci 6-3, 6-4, 3-6, 6-3.

It wasn't as breezy for other top seeds on Thursday. Australia's Jordan Thompson, unseeded but ranked 32nd in the world, rolled past No. 7 seed Hubert Hurkacz of Poland 7-6 (2), 6-1, 7-5.

Thompson hit 13 aces to just two double faults and won 42 of his 51 first-service points (82.4 percent).

Tomas Machac of the Czech Republic toppled No. 16 seed Sebastian Korda of the U.S. 6-4, 6-2, 6-4 to reach the third round of the U.S. Open for the first time.

"It was pretty difficult. I think I was playing a solid match, so was Sebi at the beginning," Machac said. "At the beginning of the third set, I noticed he was struggling, shaking out his hand, but he was still fighting so much and playing like he wasn't in pain. So I had to be really focused on my tennis."

Canadian qualifier Gabriel Diallo edged No. 24 seed Arthur Fils of France 7-5, 6-7 (3), 6-4, 6-4.

No. 10 seed Alex de Minaur of Australia took care of business against Finnish qualifier Otto Virtanen, 7-5, 6-1, 7-6 (3). No. 25 seed Jack Draper of Great Britain, No. 30 seed Matteo Arnaldi and No. 31 seed Flavio Cobolli, both of Italy, all advanced to the next round.

No. 14 seed Tommy Paul led 7-5, 6-0, 1-0 when Australian opponent Max Purcell retired.

Great Britain's Daniel Evans, two days removed from winning the longest match (five hours, 35 minutes) in U.S. Open history, made quicker work of Argentina's Mariano Navone, 6-4, 6-3, 6-3. Other winners included Nuno Borges of Portugal, David Goffin of Belgium and Jakub Mensik of the Czech Republic.