they were the fifth-ranked team and only the top four were accepted instead of eight. In their debut, the duo were upset in the first round by the lowest-seeded team of Kiki Bertens and Johanna Larsson. Individually, Barty also established a new career-high doubles world ranking of No. 11 towards end of the season. Barty and Dellacqua reached the third round at the Australian Open. This was their last WTA tournament together before Dellacqua's retirement. Barty partnered with CoCo Vandeweghe in her next three doubles events, and the pair had their best success in the United States where they won the Miami Open, Barty's first Premier Mandatory title. Although Barty continued to play primarily with Vandeweghe during the rest of the season, she also played two Premier 5 tournaments with Demi Schuurs after her usual partner Elise Mertens withdrew from the Italian Open. Barty and Schuurs won both of their tournaments together, the Italian Open and the Canadian Open. The first also helped Barty climb to a career-best WTA doubles world ranking of No. 5. Later in the season, Barty reunited with Vandeweghe and won her first career Grand Slam title at the US Open. In the semifinals, the pair defeated the top seeded team of Barbora Krejčíková and Kateřina Siniaková, who were the reigning French Open and Wimbledon champions. In the final, they defeated the second seeded team of Tímea Babos and Kristina Mladenovic, who were the reigning Australian Open champions. Barty and Vandeweghe lost the first set and were down two championship points in the second set before coming from behind to win the last two sets in two tiebreaks, saving a third championship point in the final tiebreak. This was the first time ever that a Grand Slam women's doubles final came down to a third-set tiebreak. Even though Barty and Vandeweghe only played seven tournaments together, their two big titles were enough for them to qualify as the eighth and last seed into the WTA Finals. They upset Barty's other doubles partner Schuurs, who was back with Mertens, in the first round. Their tournament was ended by Babos and Mladenovic in the semifinals in a rematch of the US Open final. 2019–2020: French Open champion, world No. 1 in singles, COVID-19 hiatus Singles: First Grand Slam title, WTA Finals champion For the second consecutive year, Barty began the season with a runner-up finish at the Sydney