As we prepare for the last Grand Slam of the season, with the US Open commencing at the end of the month, many of Serena Williams' big challengers for that title travel to the States this week to take part in the Citi Open in Washington, which begins tomorrow.

The world number one will record her first ever Calendar Grand Slam if she wins in New York, having been victorious in Australia, France and at Wimbledon already in 2015, but a number of the women in action in Washington this week will be looking to find the form necessary to challenge the great champion.

Last year

The 2014 Citi Open contained enough upsets for an entire tennis season, with defending champion Magdalena Rybarikova and top seed Lucie Safarova both crashing out in the first round, along with third seed Alize Cornet, fourth seed Sloane Stephens and seventh seed Madison Keys.

American fans in Washington did see one of their own make it to the last eight, but it was now-ranked number 308 Vania King, and she withdrew through injury before even getting the chance to face eventual champion Svetlana Kuznetsova.

Only two seeds reached the last four, Ekaterina Makarova and Kuznetsova facing each other in the semi-finals and latter coming through on top in straight sets, before she went on to defeat Japan's Kurumi Nara in three sets in the final to take home the silverware on offer.

Who's in action this year?

This year's tournament sees many of the fancied players from last year who underwhelmed return, whilst a whole new batch of contenders arrive, looking to take Kuznetsova's crown.

Makarova comes to Washington as the top seed this year, with Samantha Stosur, Belinda Bencic, Kuznetsova, Cornet, Irina-Camelia Begu, CoCo Vandeweghe and Zarina Diyas completing the seeded line-up, respectively.

Also thrown in the mix are Alison Van Uytvanck, a quarter-finalist at this year's French Open, 2014's runner-up Nara and Britain's Heather Watson, who came within two points of defeating Serena Williams at Wimbledon this year.

Representing the States are; world number 35 Madison Brengle; 23-year-old Christina McHale; the aforementioned and seeded Vandeweghe; wildcard Louisa Chirico; a disappointing performer last year in Stephens; up-and-coming 21-year-old Lauren Davis; another wildcard in Taylor Townsend and, finally, Irina Falconi, a finalist in Burnie, Australia, back in February this year.

Sanaz Marand could join this American line-up too, if she defeats second seed Elizaveta Kulichkova in qualifying today.

The Draw

There are certainly some interesting first round ties set up in Washington this year, and some intriguing possibilities for the latter stages of the tournament.

Perhaps the best of the former is that which sees top seed Makarova kick things off, as she faces Van Uytvanck in a tricky first match. Elsewhere, there are two all-American clashes between Vandeweghe and McHale and Townsend and Falconi, whilst second seed Stosur faces Kristina Mladenovic, who reached the quarter-finals here last year.

If Makarova can make it through her first match though, she has arguably the kindest run of any seed this year, with Begu to be her toughest task in the quarter-finals - albeit, one she is fully capable of handling. Promising teenager Bencic is likely to cross paths with either Nara or Vandeweghe, reigning champion Kuznetsova is drawn in the same section as Cornet, Watson and Stephens, whilst Stosur, if she overcomes Mladenovic, will probably play another up-and-coming talent in Diyas.

If all the favourites progress, the semi-finals will see Makarova face Bencic or Vandeweghe and Kuznetsova play Stosur, but, of course, this is never the case.

Favourites

Makarova is the woman to beat given that she is the highest ranked player at the tournament, but she may be able to slip into the competition under the radar after a poor showing in Wimbledon as she crashed out in round two. She has failed to break much ground this year, with her semi-final at the Australian Open in February at the back of everyone's minds now.

Kuznetsova is obviously another front runner with her being the reigning champion, but Bencic will be fancied by many given the wonderful year she is having. The 18-year-old, ranked a career high of 21st right now, recorded her best results so far at the French and in Wimbledon this year, and she claimed her first ever title on the tour in Eastbourne just before the latter, defeating Agnieszka Radwanska 6-4 4-6 6-0 to make up for the disappointment of losing in the final of the Topshelf Open just two weeks prior.

Stosur comes into the tournament having a decent year too though, the veteran having claimed two titles in 2015, including one just last week in Bad Gastein. Seeded second, she will be another player many expect a lot from.

Dark horses

There are plenty of potential underdogs in the running for the title though who will look to dismantle the favourites. Van Uytvanck is one of these and she has the chance to achieve the latter as early as the first round, where she faces Makarova. If she can come through this on top, then the Belgian could certainly stake a claim for her first ever title on the tour.

Despite being the seventh seed, Vangeweghe is a dark horse this year too, the wildcard looking for only her second WTA title and her first on hard court. She recorded best finishes at the Australian Open and Wimbledon this year and will be backed by her home crowd, so is certainly a player to watch.

Back to possible unseeded victors though, Watson could cause an upset with her being another woman having a terrific 2015. With a never-say-die attitude, the Brit bagged her second WTA title at the beginning of the year, and followed this up with her first ever win over a top 10 player when she defeated Radwanska in Indian Wells. At Wimbledon, the 23-year-old equaled her best third round finish too, defeating 32nd seed Caroline Garcia on the way, whilst nearly overcoming Serena Williams, only to lose her composure in crucial moments.

After seeing the tournament be so wide open last year though, the 2015 Citi Open is a competition anyone could win.

Prediction

Given Makarova's recent form and also that of reigning champion Kuznetsova, who had a terrible hard-court season at the beginning of 2015, neither look in good stead to emerge victorious in Washington next week.

Bencic is perhaps the best bet for a seeded winner, given her terrific year and all-round delightful talent, and she has shown she has what it takes to close games out to win titles with victory in Eastbourne.

However, Vandeweghe is certainly a good shout for the crown with her being the home favourite, and she will hope to use this to drive her to success in Washington.

Yet, one cannot look past the incredibly gifted teenager that is Bencic right now, and she has what it takes to bag her second title of the year by winning the 2015 Citi Open.