Japanese star Naomi Osaka raced into the third round of the Tokyo Olympics tennis tournament on Monday as world number one Novak Djokovic continued his pursuit of a Golden Grand Slam.

Four-time Grand Slam champion Osaka powered past Switzerland's 49th-ranked Viktorija Golubic 6-3, 6-2 in just over an hour, playing her second match in as many days after lighting the Olympic cauldron on Friday.

"Honestly, I feel like I was a bit more nervous before the match," said Osaka.

"I felt a lot of butterflies, but I think as I started playing and feeling more comfortable, I knew that no matter what it would be a great match."

Osaka won seven games in a row from 3-3 in the first set to seize control of the match at Ariake Tennis Park, where gentle breeze and slightly overcast skies made for more pleasant conditions after hot sunshine brought the extreme weather policy into play in round one.

She is the highest-ranked player left in the women's draw following the shock exit of world number one and Wimbledon champion Ashleigh Barty.

"It definitely would mean a lot for me to win gold here, but I know it's a process," said Osaka, who plays 2019 French Open runner-up Marketa Vondrousova or Romania's Mihaela Buzarnescu for a place in the quarter-finals.

"I know that these are the best players in the world, and I honestly haven't played in a while so I'm trying to take it one match at a time. But all in all I'm just really happy to be here."

Spain's Sara Sorribes Tormo backed up her defeat of Barty with a 6-1, 6-4 victory over France's Fiona Ferro.

French Open champion Barbora Krejcikova beat Canadian teenager Leylah Fernandez 6-2, 6-4, while two-time Grand Slam winner Garbine Muguruza swatted China's Wang Qiang aside for the loss of just three games.

'Best birthday present' 

Maria Sakkari, who will team up with Stefanos Tsitsipas for Greece in mixed doubles, defeated Serbia's Nina Stojanovic 6-1, 6-2 the day after celebrating her 26th birthday.

"It is one of the best presents I have given myself in 26 years that I'm alive. To spend my birthday here, I don't know if it will happen again in my life," said Sakkari.

Later on Monday, Djokovic takes on Jan-Lennard Struff of Germany for a spot in the last 16.

The Serb is the overwhelming favourite for the men's title in an event missing half the men's top 10, including 2008 Olympic champion Rafael Nadal and Roger Federer.

Djokovic, a 2008 bronze medallist, is hoping to emulate Steffi Graf's 1988 feat of winning the calendar Golden Slam after already snapping up the Australian Open, French Open and Wimbledon this year.

Alexander Zverev is a potential quarter-final opponent for Djokovic. The German strolled past Colombia's Daniel Elahi Galan 6-2, 6-2 in his second-round match.

Second seed Daniil Medvedev meets Sumit Nagal, the first Indian man to win an Olympic singles match since Leander Paes claimed bronze at the 1996 Games in Atlanta.

Aryna Sabalenka, the women's third seed from Belarus, faces Donna Vekic in the second round, with 2020 Roland Garros winner Iga Swiatek, Elina Svitolina, Petra Kvitova and Karolina Pliskova also in action.