then quarterfinals at the Premier 5-levels Canadian Open and Cincinnati Open. In Cincinnati she also made top 10 win over world No. 3, Sloane Stephens, and reached final in doubles. Then, at the Asian hardcourt swing, she won title at the Premier 5-level Wuhan Open and final of the Premier Mandatory-level China Open, both in doubles with Schuurs. At the end of the year, she played at the WTA Elite Trophy in singles and at the WTA Finals in doubles, losing in the round-robin phase, respectively. Mertens continued to progress in 2019. In February, she won her first Premier singles title at the Qatar Open by defeated world No. 3, Simona Halep, in the final. On her path to the final, she also made two more top-10 wins over world No. 8, Kiki Bertens, and No. 6, Angelique Kerber. Soon after that, she won Sunshine Doubles (Indian Wells and Miami Open), both in doubles alongside Aryna Sabalenka. She then continued with good performances in doubles, reaching semifinal of the French Open and quarterfinal of Wimbledon. At the US Open, she done well in both events, reaching quarterfinal in singles and title in doubles alongside Sabalenka. At the 2020 US Open, she reached quarterfinals for the second year in-a-row, and also came to the same stage in doubles. Mertens started 2021 season with the title at the WTA 500-level Gippsland Trophy in Melbourne, right before Australian Open. There she also defeated world No. 5 Elina Svitolina. At the Australian Open, she won her second Grand Slam doubles title, again with Aryna Sabalenka. Mertens ascended to world No. 1 in doubles, on 10 May 2021. Performance timelines 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. Singles Current through the 2023 Qatar Open. Doubles Current through the 2023 Australian OPen. Significant finals Grand Slam finals Doubles: 4 (3 titles, 1 runner-up) WTA Finals Doubles: 2 (1 title, 1 runner-up) WTA 1000 finals Doubles: 8 (4 titles, 4 runner-ups) WTA career finals Mertens made her WTA Tour debut in 2015 at the Copa Colsanitas. In 2016, she reached and won her first WTA Tour final at the Auckland Open in doubles event. In 2017, she reached her first WTA singles final at the Hobart International, where she succeeded to win the title. In 2019, she continued to progress and win