With Naomi Osaka enjoying a breakthrough campaign in 2018 and Kei Nishikori making a strong comeback from a career-threatening injury, the past season proved fruitful for emerging tennis powerhouse Japan.

Osaka started 2018 at No. 68 in the WTA rankings but saw a dramatic rise to fourth in the world -- tying the all-time record for the highest-ranked Japanese player, male or female -- in October.

She won the first title of her budding WTA career at the BNP Paribas Open in March, defeating former world No. 1 Maria Sharapova and current No. 1 Simona Halep along the way. And just days later at the Miami Open, she handed another former world No. 1 and her childhood idol Serena Williams a straight-sets loss in their first meeting.

"At the beginning of the season I didn't have confidence in myself, but now (my confidence is) stable," said Osaka, who stands 180 centimeters tall and can wind her serve up to speeds over 200 kilometers per hour.

When the 21-year-old was stunned on clay by Madison Keys in the third round of the French Open in June, she said she was told by numerous people, including her German coach Sascha Bajin, not to rely too heavily on her power, rather she should engage in extended rallies.

She took the advice and the transformation helped her achieve results, the highlight coming in September when she beat Williams in the U.S. Open women's singles final for her first Grand Slam title. Bajin also profited, as he was named WTA's Coach of the Year at season's end.

"I'm aiming to be consistent. Whether it be grass or clay, no matter what event, I want to be able to play good tennis," Osaka said.

The breakout star of 2018 received $3.8 million in prize money from the U.S. Open, more than doubling her career on-court earnings.

Her aggressive game and now-trademark reserved nature sent her off-court popularity skyrocketing, her impressive endorsement portfolio now includes major brands like Adidas, Nissin, Yonex, Citizen, Nissan and Shiseido, the value of some of the deals breaking new sports marketing ground for women.

Meanwhile, for different reasons, Nishikori also experienced a career-defining year.

Nishikori, the only Japanese male to reach the world's top four, overcame a five-month absence from the court that stretched back to last season to return to action in January.

He reflected on the moment he felt a sharp pain in his right wrist in August 2017, saying, "Honestly, I thought it was over." He opted not to have surgery, but with risk of recurrence, Nishikori said his biggest enemy was fear.

He proved he was well on the road to recovery when he captured a Challenger Tour title in Dallas in February, but in the process of his return to form he fell to as low as No. 39 in the ATP rankings in April.

The 28-year-old competed in the Madrid Open in May as an unseeded player, and he said at the time his lowly position has upside as hitting rock bottom means "there's nowhere to go but up."

In September, Osaka and Nishikori, both in the stable of sports management giant IMG, reached the U.S. Open semifinals, becoming the first Japanese duo to reach the semis of the Grand Slam tournament.

Nishikori said he felt the "switch-on moment" when he played a near error-free match against Maximilian Marterer in the first round at Flushing Meadows. "Not only am I back in the game, I'm better in some ways. I want to raise my level," he said.

Though Nishikori lost to Novak Djokovic in the final four, he reveled in his countrywoman's success, reaching out to Osaka with a congratulatory #proud tweet along with a video of her lifting the trophy.

Nishikori ended the season having ascended to the No. 9 spot even though he was unable to win an ATP Tour title, but be made the finals of three tournaments -- the Monte-Carlo Masters, Rakuten Japan Open and Erste Bank Open.

Last month, Nishikori beat Roger Federer at the ATP Finals in London, where he qualified as a replacement for the injured Argentine Juan Martin del Potro. He failed to advance after backing up from the victory over the legendary Swiss with two group stage losses.

Nishikori said he played with no pressure this season but does not expect it to be the same next year.

On Nov. 25, Osaka and Nishikori played an exhibition in Nagoya, where Osaka described Nishikori as "mysterious" in a pregame press conference. "No one on the tour ever says anything bad about him. I want to be like him," she said.

Outside of Japan's big-two, singles success was scarce, but another Japanese superstar continued to build upon his historic Grand Slam career.

In 2018, 34-year-old Shingo Kunieda maintained his grip on the men's wheelchair singles world No. 1 rank by winning his ninth Australian Open and his seventh French Open title while adding a run to the last two at the U.S. Open.

Kunieda also won both men's singles and doubles gold at the Asian Games in Indonesia this fall, victories that meant he and women's singles wheelchair gold medalist Yui Kamiji became the first two Japanese athletes to qualify for the 2020 Olympic or Paralympic Games.

At the Tokyo Games, Kunieda will be gunning for his fourth Paralympic gold.

Japanese pairings also had success in doubles.

New Zealand-born Japanese Ben McLachlan claimed two titles, the first with Briton Joe Salisbury at the Shenzhen Open in China, and then he defended his Rakuten Japan Open title with German partner Jan-Lennard Struff.

On the women's side, Makoto Ninomiya and Eri Hozumi ended the year ranked in 20th and 32nd in the world, respectively, but it was their run to the final of the French Open in June proved their season highlight.

Ninomiya and Hozumi became the first all-Japanese pairing to reach the final of a Grand Slam, but once there they were unable to make it count, losing in straight sets to the Czech duo of Barbora Krejcikova and Katerina Siniakova.

At the Toray Pan Pacific Open in September, where Osaka was beaten by Karolina Pliskova in the singles final, Nimoniya and Miyu Kato became the first pair from Japan to win the women's doubles crown.

"I'll even give up singles to reach the (doubles) summit," Ninomiya said of her goal at the Tokyo 2020 Games.