Today in U.S. Open: Federer, Wawrinka, Murray and Azarenka

9-7-15 Roger Federer
Roger Federer of Switzerland hits a return to Philipp Kohlschreiber of Germany during their match at the U.S. Open Championships tennis tournament in New York, September 5, 2015. Federer faces John Isner of the U.S. in a fourth-round game Monday evening. Mike Segar/Reuters

This year's U.S. Open is in full swing, with half the days of play behind us. As the tournament enters its second week, temperatures at the Queens, New York, courts remain high while the stakes get higher. But while Americans around the country enjoy the federal holiday, Labor Day means anything but rest for players like Roger Federer and Victoria Azarenka.

Sunday saw sisters Venus and Serena Williams defeat Anett Kontaveit of Estonia and Madison Keys of the U.S., respectively, granting each a ticket to the next round where they will face off against one another. The fourth-round win puts Serena one step closer to a calendar Grand Slam. If she succeeds in winning the Open, she'll be the first to win all four major annual tournaments in one year since Steffi Graf did the same in 1988. Later Sunday evening, No. 1 seed Novak Djokovic of Serbia beat Spain's Roberto Bautista Agut to advance to the next round.

Games continue Monday at 11 a.m. local time at the USTA Billie Jean King National Tennis Center's Arthur Ashe Stadium. The U.S.'s Varvara Lepchenko takes on Victoria Azarenka of Belarus, Stan Wawrinka of Switzerland takes on Donald Young of the U.S., Australia's Samantha Stosur faces Flavia Pennetta of Italy, Kevin Anderson of South Africa plays the U.K.'s Andy Murray, and Tomas Berdych of the Czech Republic takes on France's Richard Gasquet. Monday's evening games will see the Czech Petra Kvitova take on the U.K.'s Johanna Konta and Roger Federer of Switzerland face John Isner of the U.S.

Fans can watch Monday's matches on ESPN2 or ESPN3, sign in to stream the games online or track scores live online.