Upsets are increasinly becoming a rarity in men's tennis.

Look no further than the U.S. Open, where the top four tennis players have advanced to the semi-finals in what could be a classic tennis weekend.

Novak Djokovic and Roger Federer will meet in one semi-finals, while Rafael Nadal will face Andy Murray in the other match.

In the quarterfinals, Djokovic defeated fellow Serb Janko Tipsarevic who retired with a leg-injury. Federer earned a straight sets victory over French star Jo-Wilfried Tsonga.

On Friday, defending champion dismantled American Andy Roddick 6-2, 6-1, 6-3 in under two hours. It was another overwhelming performance by Nadal, and another setback for Roddick.

Murray defeated hard-serving American John Isner 7-5, 6-4, 3-6, 7-6, and finished off the match with 7-2 score in the fourth-set tiebreaker in a quarter-final matchup. 

The most likely outcome will be a Djokovic-Nadal final. Both Federer and Murray have had great tournaments, but it will be difficult to unseat the finalists of Wimbledon.

Federer, however, is the only player to defeat Djokovic in a match this year that was played to completion.

Murray has beaten Nadal four times in 16 matches, and has given him problems on hard courts.

When I've won against him, it's always been on hard courts. It's a good surface for me to play him on, Murray added.

In a tournament where the four favorites to make the semi-finals eventually made it, perhaps Saturday is when the upsets occur.