Four-time major winner Naomi Osaka lost in front of her home crowd to Slovakian teenager Mia Pohankova in the Pan Pacific Open round of 16 on Thursday.

Pohankova, who turned 18 last week and was a wild-card entry, upset No. 7-seeded Osaka 6-3, 7-5 to advance to the quarterfinals. Pohankova, the 2025 junior singles champion at Wimbledon, will face Alycia Parks next.

Osaka was coming off a win over Jasmine Paolini in the first round.

In other round-of-16 matches, top-ranked Elena Rybakina secured the year-end No. 1 ranking by beating Yulia Putintseva 6-2, 6-3. Rybakina won two Grand Slams this season -- the Australian Open and US Open, the latter pushing her to No. 1 in the world -- and is only a French Open title away from completing the career Grand Slam.

She described her year-end status as "an incredibly special moment for me."

"I'm really proud of what I've achieved this season and the consistency I've shown throughout the year," said Rybakina, who also won titles in Stuttgart and Ningbo.

Rybakina will be honored with a trophy to mark the achievement during a special presentation at the WTA Finals in Indian Wells from Nov. 8-15.

Leylah Fernandez defeated No. 14 Elise Mertens 6-2, 7-6 (8).

Second-seeded Maja Chwalinska and No. 4 Belinda Bencic pulled out of the tournament earlier with injuries. Chwalinska had an injured left thigh, and Bencic had a right ankle injury.

The Associated Press contributed to this report.