Sloane Stephens began preparations to defend her US Open title with a straight-sets win over Bethanie Mattek-Sands at the WTA Citi Open, where defending champion Ekaterina Makarova bowed out in Washington.

American star and second seed Stephens progressed after accounting for countrywoman Mattek-Sands 7-5 6-4 in DC on Tuesday.

Stephens saved four set points as she recovered from a 5-2 deficit to take the opener before cruising in the second – the French Open finalist hitting 40 winners in just under two hours on the hart courts.

The 25-year-old, who will face German Andrea Petkovic in the second round, earned her first career title in Washington in 2015 and is the highest-remaining seed three years later after the withdrawal of top seed Caroline Wozniacki.

Australian Open champion Wozniacki was scheduled to face Russian qualifier Anhelina Kalinina on Tuesday, but pulled out of the competition with a right leg injury.

With the US Open looming later this month, it is another blow to the world number two's preparations for the hard-court season, as she is yet to feature since a surprising loss to Makarova in the early rounds of Wimbledon.

Makarova's title defence in Washington came to an end at the first hurdle after she suffered a surprising straight-sets defeat to Romanian Ana Bogdan.

Bogdan made light work of Makarova as she broke the Russian fourth seed four times en route to a 7-6 (7-2) 6-3 win to set up a second-round clash with two-time grand slam champion Svetlana Kuznetsova.

Elsewhere, third seed Naomi Osaka beat Bernarda Pera 6-2 7-6 (7-4) and seventh seed Donna Vekic defeated Caroline Dolehide 6-3 6-4.

Belinda Bencic and Yulia Putintseva were among the other seeds to win in Washington, while there was also a straight-set victory for Nao Hibino over Sofya Zhuk before rain wreaked havoc.