Even though US Open has not ended yet, the last Grand Slam of the year has already produced some quality surprises. We can only hope there will be more, because as much as it's interesting to watch players like Serena Williams and Novak Djokovic reign the court, there's nothing quite like an out of the blue win.

Over at the men's court, last year's defending champion Marin Cilic has taught us to always expect the unexpected, and that is exactly what happened. Dark horse alert? Yes, please! These two matches had everybody's jaw on the floor:

1. Kei Nishikori vs Benoit Paire
World's #4 Nishikori came into this match as one of the favourites to win US Open singles, and came out of the court with a rookie-like lose after the first round. Benoit Paire is only 41st in the world, but he promptly defeated the Japanese tennis star by winning 3 out of 5 sets and beating incredibly high odds.

Exciting as that was though, Benoit finished his US Open in round of 16 after losing to a fellow Frenchman Jo-Wilfried Tsonga. However, four rounds at this tournament is a very solid achievement and Paire definitely managed to draw bucket loads of attention to himself. Next year, maybe?

2. Andy Murray vs Kevin Anderson
Another star fell: the third seed Andy lost in the fourth round to a relatively unknown player, Kevin Anderson. This match was undoubtedly the best result of South African's career so far, and he can now add US Open quarterfinals to his résumé after a pretty marvellous three out of four set win against the Scotsman.

One can only wonder whether he can pull off what Marin Cilic managed last year and continue the surprise champion tradition. He's about to meet Stan Wawrinka in court, and the latter has had a really good season so far: only Djokovic prevented him from winning French Open. Is Anderson a one-win wonder, or is he a latent victor?

While the women have been more consistent so far, they have also been consistently dropping like flies. On September 8th, only four of the top 20 seeds set out to the matches: mind you, it's just quarter-finals yet! There was one match in particular that stood out:

3. Caroline Wozniacki vs Petra Cetkovska
Even though Cetkovska had already defeated Wozniacki early on in 2013 Wimbledon, it was at least in the third round. Now it was a round earlier, and the Czech tennis player won two out of three sets, thus ending the Dane's US Open this year. A friendly reminder: Caroline was seeded #5 and Petra is currently WTA-ranked as 149th.

However, a dark-horse champion is out of the question: Cetkovska lost to Flavia Pennetta in the following round, although this is still her best Grand Slam result of the year. Maybe her prominent win will mean that she can get back to her best rank: three years ago, she was ranked world's 25th.

So who will win the US Open, anyway? Currently, it looks like Serena Williams and Novak Djokovic still sport the best odds before each match, and it's well known that you can count on these two to always win. Especially on Serena, who's not only about to complete a calendar-year Grand Slam – the first in 27 years – but is also the three times defending champion. The tournament ends on September 13th, so there's not much time left until we know for sure.