The Kastles won their WTT-record fifth consecutive King Trophy when they defeated the Austin Aces in the WTT Championship Match.
[2][3] On June 26, 2015, the Kastles signed Denis Kudla, a resident of nearby Arlington County, Virginia, as a substitute player.
[7] After the team announced the injury to Serena Williams, the Kastles lost a road match in which she was originally scheduled to play that same evening against the Philadelphia Freedoms, 18–16, and saw their record drop to 4 wins and 3 losses at the midway point of the season.
[8] As the Kastles embarked on the second half of their season, the team regrouped and dominated the Lobsters at home, 25–13, earning the first of what would be five straight wins.
The Kastles' big test came on July 24, when they went on the road to face the Austin Aces, who entered the match with 8 wins and only 1 loss.
The Kastles won four of the five sets for a convincing 22–14 victory that improved their record to 7–3 and clinched the team's fifth consecutive playoff berth.
A win by the Kastles would have given them control of their own destiny in the race for the WTT's top overall seed, and they would need only a home victory the following night against the Boston Lobsters.
Elina Svitolina topped Madison Brengle, 5–1, to give the Aces a 19–16 victory and the best overall regular-season record in WTT for 2015.
It was a dominant performance for Washington, as the Kastles took all five sets en route to a 25–9 victory for their fifth consecutive Eastern Conference title.
The Kastles were led by their captain, WTT Final Most Valuable Player Leander Paes, who teamed with Martina Hingis to take the opening set of mixed doubles from Teymuraz Gabashvili and Alla Kudryavtseva, 5–2, and then with Sam Querrey to win the men's doubles set, 5–3, over Gabashvili and Jarmere Jenkins.
Hingis and Anastasia Rodionova took the second set of women's doubles in a tiebreaker after each team broke the other's serve three times.
With the Kastles leading, 15–9, Querrey faced Gabashvili, who led WTT in winning percentage in men's singles during the regular season.
Brengle hit a backhand winner in the first game of extended play to end the match and secure the title for the Kastles.