She then competed at the Dubai Championships, where, as a qualifier, she beat Tsvetana Pironkova en route to the second round, where she lost to Agnieszka Radwańska.

At the Ladies Open Biel Bienne, Mertens upset Monica Niculescu and beat Mona Barthel en route to the quarterfinals, losing to Anett Kontaveit.

She defeated qualifier Viktória Kužmová, Daria Gavrilova, Alizé Cornet and Petra Martić, all in straight sets, to advance to her first Grand Slam quarterfinal.

[4][12] With her win over Svitolina, Mertens became the third Belgian woman to reach the last four at the tournament, joining former ranking leaders Justine Henin and Kim Clijsters.

She reached the fourth round at the French Open, defeating Varvara Lepchenko, Heather Watson, and Daria Gavrilova before falling again to Halep, who won the title.

Although she lost early in singles, she won a big doubles title with Sabalenka at Indian Wells, defeating first seeds Barbora Krejčíková and Kateřina Siniaková in the final.

Mertens and Sabalenka performed well in the doubles draw, losing to eventual champions Tímea Babos and Kristina Mladenovic in the semifinals.

Mertens performed much stronger in the grass-court season warmup events, making the quarterfinals in Mallorca and the third round in Eastbourne (in both tournaments she lost to the eventual champions Sofia Kenin and Karolína Plíšková, respectively).

At Wimbledon, Mertens advanced to the fourth round for the first time, taking out Fiona Ferro, Monica Niculescu and Wang Qiang.

In doubles, Mertens and Sabalenka advanced to the quarterfinals, but they were defeated by eventual champions Strýcová and Hsieh Su-wei, the second consecutive major the pair lost to the team that won the tournament.

At the US Open, she defeated Jil Teichmann, Kristýna Plíšková, Andrea Petkovic, and Kristie Ahn to reach her second Grand Slam singles quarterfinal.

Mertens opened the year by reaching the quarterfinals in both Shenzhen, where she was beaten by Elena Rybakina, and Hobart, losing to Heather Watson.

At the Australian Open, Mertens eased through to the fourth round with straight-set wins over Danka Kovinić, Watson and CiCi Bellis before being defeated by Simona Halep.

After losing early in both Dubai and Doha, Mertens was next scheduled to play in Indian Wells, but the tournament was cancelled and the tour suspended due to the COVID-19 pandemic.

She then caused a huge upset by defeating Sofia Kenin in straight sets, making it to the quarterfinals of the US Open for the second consecutive year.

[26] As the top seed in doubles alongside Kudermetova, she won the title beating Nao Hibino and Makoto Ninomiya in the final.

[38] In the process, she became the first Belgian player to win three doubles Grand Slam titles and kept her unbeaten record in major finals intact.

[59] Seeded 16th at the Qatar Ladies Open, she won her second-round match when her opponent, two-time champion Petra Kvitová, retired due to a left wrist injury.

Starting her grass-court season seeded eighth at the Rosmalen Open, Mertens lost in the second round to compatriot Alison Van Uytvanck.

She defeated Jaqueline Cristian, Despina Papamichail, Moyuka Uchijima and avenged her last loss to Claire Liu in route to the final.

In the final, she cruised past Alizé Cornet losing just two games to win her seventh WTA Tour title in Monastir.

[83][84] In Fort Worth, Mertens along with Kudermetova raced into the doubles final, as they finished 3–0 in the round-robin stage and eased past Desirae Krawczyk and Demi Schuurs in the semifinals.

At the Madrid Open, she also reached the fourth round with wins over Rebecca Marino and Jule Niemeier but lost to Mayar Sherif.

[89][90] At the French Open, she reached the fourth round with wins over lucky losers Viktória Hrunčáková, Camila Osorio, and third seed Jessica Pegula, before losing to Anastasia Pavlyuchenkova.

[98] Partnering with Hsieh, Mertens won the Indian Wells Open in March, defeating former world-number-one players Hunter and Siniaková[99][100] and the Birmingham Classic in June, with victory over Miyu Kato and Zhang Shuai.

[103][104] Alongside Hsieh, Mertens qualified for the end-of-season WTA Finals but exited in the group stages after compiling a record of one win and two losses.

[105] As second seed, Mertens reached the final at the Hobart International, defeating Nuria Párrizas Díaz,[106] Renata Zarazúa,[107] Veronika Kudermetova[108] and Maya Joint.

Her groundstrokes are hit flat, with little topspin, allowing her shots to penetrate consistently deep into the court despite Mertens' comparatively slight build.

Mertens' superlative fitness, stamina, speed, footwork, and court coverage allow her to excel at counterpunching, and extend points until she creates the opportunity to hit low-risk winners; as such, she is one of the most effective players on the WTA Tour at turning defence into offence, due to her excellent point construction.

Mertens possesses extreme mental toughness, and has been noted for her consistency and determination on court, making her a formidable opponent.