[2] Doi reached two junior Grand Slam doubles finals – at Wimbledon in 2007 with Kurumi Nara, and at the Australian Open in 2008, with Elena Bogdan (losing both).

In 2007, while enrolled as a freshman in Sundai Kōei High School, Doi earned second place in the Japan Open Junior Championships in Nagoya.

They placed second in girls' doubles at the 2007 Wimbledon Championships, becoming only the second Japanese pair to reach the finals of a Grand Slam juniors event since Yuka Yoshida and Hiroko Mochizuki at the 1993 US Open.

Doi and Nara continued their run by advancing to the girls' doubles semifinals at a number of high-profile tournaments, such as the 2007 US Open and Wimbledon 2008.

She partnered with Kurumi Nara again for the 2008 ITF event in Miyazaki, where they upset top-seeded sisters Erika and Yurika Sema, 3–6, 6–3, [10–6] in the second round.

In doubles, she recorded three second-place finishes in as many weeks in April tournaments at Incheon, Gimhae, and Changwon, South Korea, with partner Junri Namigata.

With new partner Kotomi Takahata, Doi won her first $50k title in doubles at the Fukuoka International in May, defeating Marina Erakovic and Alexandra Panova in straight sets.

With her victory over Vitalia Diatchenko, Doi had earned a spot in her first major tournament main draw, where she lost to Polona Hercog in the first round.

[7] The Birmingham Classic was Doi's first appearance in the quarterfinals of a singles tour event, which she reached by defeating the top seed Francesca Schiavone in two sets.

Although Doi lost in the Wimbledon qualifiers to Kristina Mladenovic, she received a lucky loser berth in the main tournament.

She then reached the last 16 of Wimbledon, beating Louisa Chirico, Karolína Plíšková and Anna-Lena Friedsam before losing to Kerber, in straight sets.

Doi was the first Japanese player to reach the fourth round of the ladies draw since Ai Sugiyama ten years earlier.

She had a minor resurgence on clay, beating WR13 and ninth seed Madison Keys and qualifier Donna Vekic at the Mutua Madrid Open, and then reaching the semifinals at Nürnberg.

For the rest of the year, Doi was not able to win back-to-back matches in tournaments, including a first-round exit in the US Open to Barbora Strýcová.

She beat Tamara Zidanšek in the first round of qualifying at the Brisbane International before falling to Heather Watson in straight sets.

After failing to qualify for Charleston, Doi played a number of ITF events with very limited success, winning only two matches and losing five.

Doi then had a breakthrough in her season, by qualifying and winning a $100,000 ITF event in Vancouver, beating Heather Watson in the final.

This provided her ranking a boost of around 80 places and meant she could play the qualifying event of the US Open - but lost in the first round to compatriot Nao Hibino, 4-6, 3-6.

After a first-round loss as a wildcard in Hiroshima, Doi qualified for the Pan Pacific Open before falling in the first round to Camila Giorgi.

Her form took a turn in Indian Wells, where she beat Francesca Di Lorenzo and Yanina Wickmayer to qualify, before defeating Ons Jabeur (WR55) in straight sets in the first round.

She repeated the feat in Miami, beating 71st-ranked Anastasia Potapova and Mandy Minella in qualifying, before defeating wildcard Wang Xinyu in the first round.

Doi then suddenly turned around her poor form with a run to the title in Bastad with wins over Australian Alison Bai, seventh-seeded Kazakh Elena Rybakina, fourth seeded German Mona Barthel, eighth-seeded Serbian Aleksandra Krunić, before beating Montenegrin Danka Kovinić in the final.

Her final tournament of the season resulted in a three-set first-round loss to second seed Julia Görges in the Luxembourg Open.

She entered the main draw of the Qatar Open as a lucky loser, but lost for the second time to Tereza Martincová, who had beaten her in qualifying.

After a second-round appearance in the Istanbul Cup, Doi qualified for Rome, beating Martina Trevisan before falling 1-6 6-4 4-6 to 12th seed Markéta Vondroušová in the opening round.

Only main-draw results in WTA Tour, Grand Slam tournaments, Fed Cup/Billie Jean King Cup and Olympic Games are included in win–loss records.