Zverev beat the 19-year-old Australian 6-2, 6-4 in the warmup event for the US Open to claim a third ATP title this year after wins in Munich and Madrid, BBC Sport reported.
"This match could be the final for the next 15 years so I hope you really enjoyed it," said the German, 21.
In the final of the women's event, Russian Svetlana Kuznetsova beat Croatia's Donna Vekic 4-6, 7-6 (9-7), 6-2.
It was an 18th career tour title for the 33-year-old two-time Grand Slam champion, who had slipped to 128th in the world heading to Washington.
Elsewhere, Mihaela Buzarnescu won her first WTA title with a 6-1, 6-0 victory against Greece's Maria Sakkariat at the Silicon Valley Classic.
The 30-year-old wrapped up the final in just 73 minutes and has now broken into the world's top 20.
Meanwhile, Fabio Fognini beat world number four Juan Martin del Potro 6-4, 6-2 to win the Los Cabos Open.
Italian Fognini, 31, had beaten British number two Cameron Norrie to reach the final in Mexico.