Perhaps the longest calendar in professional sport, professional tennis is a marathon rather than a sprint.

A WTA season that boasts nearly 60 events over 10 months on five continents is exhausting to fathom, let alone compete in. With the four majors in the rearview mirror, the finish line lies in sight. That finish line is the WTA Championships in Istanbul, played annually at a rotating site as the climax of the WTA season. Generally considered the most prestigious event on tour following the four majors, it also offers the most prize money and ranking points of any non-major.

Last season, Li Na became the final player to qualify for the WTA Championships on October 5, just one day after Angelique Kerber, Petra Kvitova and Sara Errani all qualified. As that date approaches one year later in 2013, the scene looks very different.

At the start of the week in Beijing, the year’s last Premier Mandatory tournament, only five players had qualified for Istanbul. Serena Williams had qualified for the event after Roland Garros, Maria Sharapova after Toronto, and Victoria Azarenka after Cincinnati.  Agnieszka Radwanska and Li Na booked their tickets to Istanbul after the US Open.

Including this week, there are just five tournaments left before the season-ending event begins. With Sharapova's health status in doubt, it is entirely possible that four rather than three spots remain open even at this stage in the Race to Istanbul. As a result, the players still in contention have the chance to take late-season form and run with it.

While the slumping Sara Errani clings to sixth place in the Race, Petra Kvitova has led a late-season surge by some of the Istanbul hopefuls. The former Wimbledon champion scored a key breakthrough by winning the WTA Championships in 2011. (At the time, many saw that title as a launch pad from which she could vault to the top of women's tennis, but events have unfolded otherwise.) Having triumphed in Tokyo, Kvitova seems poised to reach her third straight Championships despite having reached the second week at just one of four majors in 2013.

Angelique Kerber had looked like another victim of a sophomore slump this year after her 2011-12 exploits.  And yet she too may have salvaged her Istanbul hopes at the last moment. An unexpected run to the Tokyo final, built on an upset of Agnieszka Radwanska, vaulted Kerber from No. 12 to No. 8 in the Race.

Jelena Jankovic, Roberta Vinci, Sloane Stephens and Caroline Wozniacki, all of whom have recorded inconsistent results in 2013, remained alive in the Race as play started on Wednesday in Beijing. Each of these women still has a chance to end her season on a high note, emulating Kvitova and Kerber.

In the doubles competition, the race is even tighter and surrounded by even more questions. Not only will this year's WTA Championships be the final edition to be staged in Istanbul, it also will be the last to feature a four-team, single-elimination doubles format. Next season, the doubles field will expand to eight teams and a round-robin format parallel to the singles field.

With Sara Errani and Roberta Vinci having qualified earlier this month, four teams are competing for the remaining three spots.

Katarina Srebotnik and Nadia Petrova have only played one event together since Wimbledon, for Petrova has suffered from a litany of injuries. Srebotnik has spent the majority of her summer and fall playing with Jelena Jankovic, a partnership of modest success so far. Ekaterina Makarova is also struggling with injuries, forcing her usual partner Elena Vesnina to partner Mandy Minella last week and sit out the doubles action this week.

Hsieh Su-wei and Peng Shuai, the Wimbledon champions, could have secured a spot had they won the title in Tokyo last week. But they fell in the semifinals there to Cara Black and Sania Mirza. Ashleigh Barty and Casey Dellacqua are on the outside looking in despite having reached three major finals this season. Partially hindered by Barty's play restrictions under the Age Eligibility Rules, the pair must make the final this week in Beijing to retain any hope of qualifying.

Typically, the compelling narrative of the WTA Championships comes from a battle for the year-end No. 1 singles ranking, or to determine the WTA Player of the Year. Since Serena Williams has locked up both achievements with her stellar play this season, the journey to qualify for Istanbul offers the most compelling story in the final miles of the WTA's marathon season.