Rising Japanese superstar Naomi Osaka has vowed to push through fatigue and atone for her error-strewn loss to Sloane Stephens in her WTA Finals debut in Singapore on Monday.

An error-prone Osaka struggled with her rhythm and faded badly against Stephens, who won 7-5, 4-6, 6-1 in two hours and 25 minutes.

The 21-year-old came into the season finale as the in-form player, having won 14 of her previous 16 matches and capturing the US Open.

It has been a whirlwind for Osaka since her stunning victory over Serena Williams at New York’s Flushing Meadows in September.

“I haven’t gone home [to Florida] since the US Open. So of course I’d love to go home, but I’m here right now and I want to do well,” she said.

“I want to give them [fans] a good match to watch. So for me that’s my main priority, and I’m not really tired of that yet. So I just want to keep seeing how far I can go.”

The world number four was visibly frustrated several times and dropped her racquet in disgust after failing to close out the second set in the ninth game.

Osaka, who had 46 unforced errors, said she hoped to keep her emotions in check moving forward.

“I am a little bit apologetic for doing the stuff that I did,” she said. “I feel like for me I play better when no one knows what I’m thinking. I think I learned a lot from today.”

Osaka has experienced a surge of popularity since her Grand Slam breakthrough and had the crowd at the Singapore Indoor Stadium on her side.

“It was really awesome. I wasn’t really expecting it,” Osaka said of her support. “I heard a lot of cheers. I’m just really grateful.”

In an unpredictable encounter, the American overcame a second set stutter to dominate the decider and triumph 7-5, 4-6, 6-1 in two hours and 25 minutes.

Stephens handled the warm conditions better to record an impressive victory.

“It’s really humid inside the stadium, so it plays a little bit different,” she said.

“Obviously playing someone with a lot of confidence is always a tough task, but I think that I played well and I kept a good attitude.”

It was followed by another marathon match with number eight seed Kiki Bertens overcoming top-seed Angelique Kerber in a thrilling 1-6, 6-3, 6-4 comeback victory.

The lower seeds have won all four matches to start the WTA Finals.

In the later match, Bertens incredibly flipped the script after being a set and a break down. The Dutch player went for broke and ran the Wimbledon champion around the court to take control of the contest.

The third set started with six consecutive breaks before Bertens pulled away to cap off a victory. Her aggressive approach saw her have a decisive 33-11 edge in winners.

“I had a slow start but I’m really happy to have turned this match,” she said. “I had a chat with my coach and we decided for me to go for my shots and play more aggressive.”

Bertens qualified just days before the tournament after world number one Simona Halep pulled out due to injury, which bumped up Kerber to top billing.