The 2013 U.S. Open is underway and on Day 2 of the lengthy tournament some of tennis' heavyweights will be in action. Live stream all the U.S. Open action here for free.

On the men's side, world No. 1 Novak Djokovic and No. 7 Roger Federer, the two biggest names in the sport will kick off their quests. Djokovic will be matched up against Lithuanian Ricardas Berankis, who has a career record of 41-36, and is an even .500 this year in singles play. Federer, meanwhile, will take on Grega Zemlja of Slovenia, who enters the U.S. Open at 13-19 this season.

They will try to open up their tournaments as impressively as Rafael Nadal did on Day 1, winning 6-4, 6-2, 6-2 against Ryan Harrison and improving his record to 16-0 on hard courts in 2013. According to ESPN, some attribute Nadal's dominance on the hard courts to his volatile and oft-injured knees.

Nadal has played with increased urgency, knowing that his knees could act up at any time. With that in mind, his play has been supremely aggressive and difficult for opponents to handle.

"He's playing maybe the best tennis that he ever has played on hard courts," said Djokovic "He has changed a little bit the game. He stepped in a little bit more. He knows that now he has to be a bit more aggressive than he usually is because of, I guess, his knees. It's not his favorite surface; it's faster. I'm sure he worked on that and you could see all the work he put in [is] getting results."

Nadal is closing in on a No. 1 ranking by season's end, but is holding his horses on that talk. "This tournament will make the difference. Is true that I am having a great season. I am in this position because I played amazing."

On the women's side Serena Williams was unsurprisingly dominant on Day 1. Today, No. 2 Victoria Azarenka takes on Dinah Pfizenmaier, No. 4 Sara Errani faces Olivia Rogowska, and No. 6 Caroline Wozniacki battles Ying-Ying Duan.