TOKYO (AP) -- There were never any questions about Naomi Osaka's potential on the tennis court when she took a two-month mental health break.
Especially on hard courts -- the surface being used at the Tokyo Olympics and the surface on which Osaka has won all four of her Grand Slam titles.
The Japanese superstar crushed winners off both wings -- forehand and backhand -- seemingly at will in a 6-3, 6-2 win over 49th-ranked Viktorija Golubic of Switzerland on Monday to reach the last 16 at the Ariake Tennis Park.
It was similar to the way Osaka overwhelmed Zheng Saisai of China a day earlier and it extended her momentum after carrying out the ultimate honor at the Games' opening ceremony by lighting the Olympic cauldron.
Osaka was asked in March to handle the cauldron honors but said she "didn't feel pressure" about the assignment.
"I felt more excitement," Osaka said. "It was like a sense of duty, like something I wanted to accomplish.
"It's something that you see as a kid on TV. You gather around the TV with your family at the Olympics and you watch the whole ceremony," Osaka added. "I know my grandparents were probably crying and my mom of course."
This is Osaka's first event since she withdrew from the French Open in May, revealing that she has dealt with depression. She then sat out Wimbledon.
Two more wins and Osaka will be in line for more honors in her Olympic debut -- a medal.
"Definitely it would mean a lot for me but I know it's a process," Osaka said. "The flag is next to my name no matter what tournament I play but I feel like the scale of this is much bigger. It's something I've been waiting for for eight years (since she turned pro in 2013)."
The second-ranked Osaka will next face 2019 French Open runner-up Marketa Vondrousova of the Czech Republic, who beat Mihaela Buzarnescu of Romania 6-1, 6-2.
A couple of other top women struggled.
Donna Vekic of Croatia beat third-seeded Aryna Sabalenka of Belarus 6-4, 3-6, 7-6 (3) and Alison van Uytvanck of Belgium rallied past 2016 bronze medalist Petra Kvitova of the Czech Republic 5-7, 6-3, 6-0.
In men's action, second-seeded Daniil Medvedev of ROC and fourth-seeded Alexander Zverev of Germany each advanced quickly. Medvedev beat Sumit Nagal of India 6-2, 6-1 and Zverev eliminated Daniel Elahi Galan of Colombia 6-2, 6-2.
Later, top-ranked Novak Djokovic of Serbia was to play 48th-ranked Jan-Lennard Struff of Germany in his quest for a Golden Slam.
Meanwhile, the International Tennis Federation announced that Dutch player Jean-Julien Rojer tested positive for COVID-19 and was withdrawn from the doubles tournament with partner Wesley Koolhof.
The eighth-seeded pair were scheduled to play Marcus Daniell and Michael Venus of New Zealand. Daniell and Venus received a walkover into the quarterfinals and Rojer was placed in isolation.
Also advancing on the women's side were seventh-seeded Garbine Muguruza of Spain, French Open champion Barbora Krejcikova of the Czech Republic and No. 9 Belinda Bencic of Switzerland.
In windier, cooler and more overcast conditions than the sweltering opening two days of the competition, Osaka compiled 29 winners to Golubic's 14 and had only 11 unforced errors to her opponent's 21.
Perhaps more telling was that Osaka required only 65 minutes to dispatch a player coming off a quarterfinal appearance at Wimbledon.
Osaka took control with a break for a 5-3 lead in the opening set, slapping her left thigh before returning serve to get herself motivated and then crushing a forehand cross-court winner to conclude the game.
Two more breaks followed at the start of the second set and Osaka cruised from there.
While the Ariake Coliseum is empty of spectators like all events at the Tokyo Games because of the pandemic, the photographers' pit during Osaka's matches is busy as ever with her every move documented.
On her first match point, Osaka hit a big second serve down the T then rushed forward to meet Golubic's weak return with an inside-out forehand winner.
With her work for the day done, Osaka clenched her left fist and glanced back at her support team behind the court with a grin -- as the photographers clicked away.