The Americans downed the Czech Republic 4-1 in their opening tie and needed just two match wins from their tie against Germany to seal top spot.
Fritz proved too much for Alexander Zverev winning 6-1, 6-4 to give the United States the edge in the first match of the day. He moved well and his powerful serve proved enough to give Fritz the leg up in the clash with the German No 1 who is returning from a long injury layoff.
Fritz was quickly on to the front foot and raced out to an early 5-0 lead in the first set before keeping the heat on to win both the first stanza and the match.
It was the American's third victory in seven matches against Zverev who has been out of action since June.
With America on top early Keys stepped up to down Jule Niemeier 6-2, 6-3 and ensure that they booked their final spot.
Keys has looked in good touch during the United Cup and her power serve proved too much for another servebot in Niemeier.
Serving at 77%, Keys lost just six points on her renowned first serve and faced no break points throughout their 71-minute match in Sydney. Niemeier recorded seven double errors throughout the course of the game. The German fought bravely in the decisive games, holding off Keys in a seven-minute game to keep the second set at 4-2 before breaking Keys on her only break point of the match.
"I think overall I played really solid," Keys said. "I just tried to make as many first serves as possible to make things as easy as a tennis match could be."
The Group C tie concludes on Tuesday, when the USA will be out to seal the win and stay unebaten in Group C.
American women's No 1, Jessica Pegula will also be looking to register her first win of the season against Laura Siegemund.
Frances Tiafoe will face Oscar Otte in the fourth and final singles match while the mixed doubles lineup has not been confirmed.
The USA will go on to face Great Britain who beat hosts Australia in their first tie of the tournament before a massive upset victory over Spain.
During that tie Cameron Norrie powered past Rafael Nadal for his first victory over the former World No 1.