World No. 8 Alexander Zverev defeated South Africa's Kevin Anderson in straight sets to clinch the Citi Open men's singles title.
Zverev defeated Anderson 6-4, 6-4 and captured his fourth title of the year in the finals played on Sunday.
It was Zverev's first win in Washington DC, and he did it without facing a single break point. He also became the youngest Citi Open champion and the youngest player to win four titles in a season since Juan Martin del Potro, who did both in 2008.
"I think I improved a lot in the past few months and in the offseason, so I can be able to win these tournaments," said the 20-year-old.
"I showed it in Rome, I showed it here again. I hope I can continue to play like this or even better."
Meanwhile, Russia's 58th-ranked Ekaterina Makarova came from a set behind against German fourth seed Julia Goerges to win the women's singles title.
Makarova defeated Goerges 3-6, 7-6 (2), 6-0 to claim her first WTA title in more than three years.
"I have such great memories from when I was a kid about Washington, so I was always dreaming of winning this title," Makarova said after the match.
Keys wins all-American final to claim Stanford title
Madison Keys outduelled fellow American Vandeweghe 7-6(4), 6-4 on Sunday to win the Stanford Classic and capture her first title of the year.
In an evenly matched contest between two friendly opponents, third seed Keys was impregnable on serve but claimed the only break in the second set before closing out a confidence-boosting win ahead of the US Open.
Keys missed the opening two months of the season after undergoing surgery on her left wrist but has worked her way back into solid form, knocking out Wimbledon champion Garbine Muguruza in the semi-final.
"It hasn't been the easiest season, but it finally feels like I'm on the right track and feeling good about my tennis again," Keys told reporters.
"It feels amazing to have a title at home and on hard courts. I'm really, really happy that this is kind of the start of my US Open Series."
Vandeweghe, who reached the semi-finals of the Australian Open and the quarter-finals at Wimbledon this season, was also looking for her first title of 2017.
She blasted six aces but faltered in the opening set tie-break to allow Keys to take control.
"I think it was a high quality match from both players. It was fun to play a friend, to have an all-American final - it was enjoyable," Vandeweghe said.