Serena Williams was challenge much more in her second match of the US Open than she was in the first. Her victory over Kiki Bertens showed some nerves from the tennis star as she was down at one point before finally winning a break and going on to take the second set with ease. Her next opponent is a fellow American aiming to be the player that takes down Williams in her quest for the calendar grand slam. Bethanie Mattek-Sands has all of the confidence to make it happen.

Mattek-Sands might not be a household name, but she certainly has the confidence to go up against the best in the sport. Williams has beaten her twice before in their career, but Mattek-Sands believes she is playing some of her best tennis right now which could poise to push her for the upset win.

"She's playing great. Right now she's on a mission to get a record, and I'm here playing my game," Mattek-Sands said, via Tennis.com. "I think it's going to be a battle out there. I'm going to do my best. Obviously Serena's pretty aggressive. She's going to rip some balls, hit some big serves. I think it's going to be a good match. That's obviously going to be the clash there. I'm excited for it."

Williams knows there is a target on her back. The star is approaching this tournament by not thinking about the potential history she could make with a win. Getting the second round victory helped shake out lingering nerves Williams had and now she says she is ready to really dig in to the tournament games now with the talent and competition increasing.

"Knowing that she's capable of having big wins kind of relaxes me because I know she's going to come out and I know what to expect. She's going to give 300 percent. She's a huge fighter," Williams said, via the New York Times. "She has a great game, by the way. I know that will help me, that I have to start out strong if I want to stay in the tournament. If not, I can go on vacation."

Pressure continues to increase for Williams. The further she gets in the tournament, the closer to history she is. With expectations mounting, the match against a tough opponent like Mallek-Sands will push the star to another limit. 

Watch Serena Williams vs. Bethanie Mattek-Sands live stream, click here.