(London, England)  --  Americans are making waves on the first day at Wimbledon.  Frances Tiafoe pulled the first major upset of the tournament on the men's side by knocking off fourth-seeded Stefanos Tsitsipas in straight sets.  The 23-year-old Hyattsville, Maryland, native won 6-4, 6-4, 6-3. On the women's side, 28-year-old Sloane Stephens upset two-time Wimbledon champion Petra Kvitova 6-3, 6-4 on Centre Court.  Today, Roger Federer and the number-two seeded Daniil Medvedev will be in action while Americans Alison Riske, Venus Williams and Serena Williams will all be on the court.