The top seeds from both the men's and women's sides remain active in the 2013 U.S. Open, but that doesn't mean they're safe from the competition.
Novak Djokovic and Serena Williams are dominating right now and have cruised their way to this point of the tournament. It's going to take a big performance from an opponent to knock either one off.
The players on this list are going to be pretty obvious, but they are the only ones who stand a chance against either Djokovic or Williams.
With only a few rounds remaining, here are the players who can challenge the top seeds.
No. 3 Andy Murray
Assuming everything goes as planned, Djokovic and Andy Murray will meet in the semifinals.
Murray must first get past No.9 Stanislas Wawrinka in the quarterfinals, and Djokovic will have to beat No. 21 Mikhail Youzhny.
Djokovic is on a good run in this tournament. He hasn't dropped a set through four rounds, and opponents are struggling to earn points against him. He has kept opponents off the board in three of the last six sets.
Murray has dropped two sets in the tournament, but he's looked good as well. The 26-year-old has also had some success against the top player in the world in recent matches.
In last year's U.S. Open, Murray defeated Djokovic in five sets to win the title. Murray again topped Djokovic in straight sets to win the 2013 Wimbledon.
The two have split their last 10 meetings, and Djokovic holds a slight 11-8 lead in the series.
Murray needs to take care of Wawrinka without exerting himself too much. Djokovic is going to be well rested, so Murray can't afford to be worn down in the quarterfinals.
No. 2 Rafael Nadal
Here's the player with the best chance of knocking off Djokovic. Picking the No. 2 player doesn't take much courage, but it's the most likely to happen.
Rafael Nadal has lost only one set at the 2013 U.S. Open, and he should be fresh entering the finals. He gets No. 19 Tommy Robredo in the quarterfinals and would face either No. 4 David Ferrer or No. 8 Richard Gasquet in the semifinals. Both Ferrer and Gasquet were pushed to the limit in the fourth round.
Nadal looks like the player to beat. He has defeated Djokovic in consecutive matches this year and has won five of the six meetings dating back to 2012.
Djokovic had won four straight matches against Nadal on hard surfaces, but Nadal snapped that streak in their last meeting.
The two have had gritty battles in their careers, and Nadal owns a 21-15 advantage in the series. Nadal owned the early part of the series, and then Djokovic started taking care of business. However, Nadal has responded by owning Djokovic in the last two years.
If the two were to meet in the finals, it would be a great battle. Nadal has the best chance to beat Djokovic, so stay tuned.
*All men's singles information is courtesy of ATPWorldTour.com
No. 2 Victoria Azarenka
Williams only has two opponents left after cruising through the quarterfinals, with match statistics courtesy of the official website of the U.S. Open.
Williams will play No. 5 Li Na in the semifinals. Williams is 8-1, including seven straight victories, in her career against Li. The top player in the world has yet to lose a set in the tournament, and her opponents have managed only one point in three sets.
Li won't be the player to defeat Williams. If anyone is going to do it, it will have to be Victoria Azarenka. It's not simply because she is No. 2; it's because she has been able to beat Williams this year.
The two players have met in three finals this year, and Azarenka has won two of those meetings. Both of Azarenka's victories have come on hard surfaces, and Williams won on clay. Williams owns the overall series 12-3, but Azarenka has found a way to beat her rival this year.
Both players will be rested entering the finals, and it will be interesting to see if Azarenka's recent success will continue at the 2013 U.S. Open.
*All women's singles information is courtesy of WTAtennis.com