American duo Pegula and Gauff are the first Americans to feature at the WTA Finals in both singles and doubles since Serena and Venus Williams in 2009.
Partners on the doubles court, Pegula and Gauff have enjoyed tremendous success this season, together and apart.
In singles, they became the first American pair to be ranked simultaneously in the top four since the Williams sisters in 2010.
In doubles, they reached the Roland-Garros final as a team and won three titles, in Doha, Toronto and San Diego. Gauff even reached the No.1 ranking in doubles earlier this season.
"I would say she has really good energy," the 28-year-old Pegula said of the teenaged Gauff.
"A lot of first-pumping, jumping around. Her athleticism and the youthfulness about her. She's very giggly. She laughs a lot. She's always kind of messing around.
"I think just having that kind of energy, especially when we're playing doubles, has kind of taught me to kind of do that more in my singles game since I'm pretty laid back, but I think at times I've picked up little things from that. So, yeah, I don't know, she keeps me young," added Pegula with a laugh.
Pegula is coming off the biggest title triumph of her career, having lifted the WTA 1000 trophy in Guadalajara last week.