World number two Aryna Sabalenka followed Elena Rybakina out the door at the China Open as she fell to a surprise defeat by 34th-ranked Nikola Bartunkova.

It comes a day after top seed Rybakina, who defeated Sabalenka in last month's US Open final and replaced the Belarusian at the top of the rankings, was knocked out by Armenian world number 118 Alina Charaeva.

Czech Bartunkova sprung another upset at the WTA 1000 tournament and continued her breakthrough season with a 6-4 6-3 over Sabalenka.

"I focused on myself, to be calm," said Bartunkova, who will face Austria's Sinja Kraus next.

"She's a legend, so I'm really happy that I won the match."

It is the first time since 2015 that the top two seeds have failed to reach the round of 16 at the China Open.

Sabalenka has not won a title since completing the 'Sunshine Double' in March, when she triumped at Indian Wells and the Miami Open back-to-back.

It has been a mixed year for Sabalenka by her high standards. She will finish without a Grand Slam trophy for the first time since 2022 and has often struggled to control her frustrations on court.

Against Bartunkova, she failed to convert any of her five break points and she threw her racquet after falling a break down early in the second set.

At the Japan Open, Carlos Alcaraz reached the semi-finals after Denis Shapovalov retired with an injury to his right thigh.

Alcaraz, the defending champion in Tokyo, saved a set point in the opening set and was leading 7-6 (7-3) 2-1 when the Canadian was forced to withdraw.

It is just Alcaraz's third event since returning from a wrist injury at last month's US Open, where he fell to Ben Shelton in the quarter-finals.

He will face fellow Spaniard Jaume Munar for a place in the final.