Tokyo Olympics: Favourites Naomi Osaka, Novak Djokovic sail into third round of respective Games tennis tournaments

Japanese medal hope Naomi Osaka sailed into the third round of the Tokyo Olympics on Monday, as some of her closest competitors in the women's singles crumbled under pressure and crashed out of the tournament.

World No.3 Aryna Sabalenka of Belarus fell to Donna Vekic in a thrilling three-set encounter on Centre Court, with the Croatian prevailing 6-4 3-6 7-6(3).

Sabalenka's loss came a day after top-ranked Australian Ash Barty stumbled out of the women's singles event in a lopsided defeat to Spain's Sara Sorribes Tormo.

Two-time Wimbledon champion and the Czech Republic's Olympic flag-bearer Petra Kvitova was beaten by Belgian Alison van Uytvanck 5-7 6-3 6-0, while last year's French Open champion Iga Swiatek also fell 6-3 7-6(4) to Spaniard Paula Badosa.

Osaka, who returned to competition this week after a two-month break, has been in impressive form while embracing her role as one of the host country's most prominent ambassadors of the Games.

Winning Olympic gold on home soil would be the icing on the cake, though Osaka said she was determined not to look too far ahead.

"Well you know, definitely it would mean a lot for me to win gold here but I know it's a process," the world number two said.

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

Osaka had not played a match since withdrawing after the first round at the French Open in May, amid controversy over her decision to skip mandatory news conferences during the tournament in a bid to protect her mental health.

The four-time Grand Slam singles champion was relentless in her match against 50th-ranked Swiss Viktorija Golubic, securing the 6-3 6-2 win with a powerful forehand to set up a meeting with Marketa Vondrousova of the Czech Republic.

Some other title contenders also advanced.

French Open champion Barbora Krejcikova continued her run of good form, taking out Canadian teenager Leylah Fernandez 6-2 6-4, while two-time Grand Slam winner Garbine Muguruza made short work of China's Wang Qiang, winning 6-3 6-0.

Top-ranked Djokovic, who is attempting to become the first man to win all four Grand Slam titles and Olympic gold in the same calendar year, had a 6-4 6-3 win over Jan-Lennard Struff of Germany in the men's singles.

The players were evenly matched at the start of the first set but the 20-time Grand Slam champion proved too strong, pouncing on Struff's serve while leading 5-3 to take the first set.

An early break in the second secured the victory.

"It was a tough opponent for a second round... but you know, I'm very pleased with the way I felt on the court," Djokovic told reporters.

Struff's compatriot and fourth seed Alexander Zverev brushed aside Daniel Elahi Galan of Colombia 6-2 6-2, while world number two Daniil Medvedev breezed through with a comfortable 6-2 6-1 win over India's Sumit Nagal.