A is for age-defying. 
Serena Williams was 33 years old when she won the 2014 title in Singapore, making her the oldest champion in the tournament's history. Another record was set by a Williams sister in the city - Venus was 37 years old when she reached the final in Singapore last year, becoming the oldest finalist in the tournament's history. 

B is for Babos. 
In the 2017 final, Timea Babos and her partner Andrea Sestini Hlavackova came from a set down against Kiki Bertens and Johanna Larsson to land the WTA Finals doubles title for the first time. Over the last four years in Singapore, the doubles title has been won by four different teams. 

C is for countries. 
The eight singles players at last year's WTA Finals were each representing a different country: Romania, Spain, Czech Republic, Ukraine, United States of America, Denmark, Latvia and France. That was the fifth time that had happened in the history of the tournament, which goes back almost half a century to 1972. 

D is for debutante
In 2016, Dominika Cibulkova became the first debutante to win the WTA Finals since Petra Kvitova in 2011. In the history of the tournament, the only other two women to have won the title as a debutante are Serena Williams (2001) and Maria Sharapova (2004). 

E is for eight. 
That's Caroline Wozniacki's "lucky number". "One of the reasons is that if you turn eight on its side, it's the infinity sign. I kind of like that. I think that's cool." The first seven times that Wozniacki played Venus Williams, she lost. On their eighth meeting, in the title match of last year's WTA Finals, the Dane defeated the American for the first time. That gave Wozniacki the biggest title of her career to date, setting her up for her first Grand Slam triumph at the 2018 Australian Open. 

F is for fifth. 
Victory in 2014 gave Serena Williams a third successive title at the season-ending tournament, and her fifth overall. putting her level with Stefanie Graf. Only Navratilova has more WTA Finals singles titles, with eight. 

G is for games. 
In the 2014 final, Serena Williams gave up just three games against Simona Halep. That was quite a turnaround from their meeting in the round-robin stages of the same tournament, when Halep had defeated Williams for the loss of just two games. If that group match had been "embarrassing" for Williams, in the final she summoned up what she called "Serena-style tennis". 

H is for haircut. 
Cutting your own hair is risky at the best of times, when you're away from the tennis court and sitting at home in front of a mirror; it is even more perilous when you are midway through a round-robin match at the WTA Finals. But, at the 2016 tournament, there was the sight of Svetlana Kuznetsova hacking away at her ponytail during a changeover. The hair had been a nuisance, hitting her in the face. "I was struggling. I thought to myself, 'OK, what's more important now, my hair, which I can grow, or the match?' I thought, 'OK, I got to go for it right now'." Kuznetsova would go on to beat Agnieszka Radwanska. 

I is for inches. 
On her run to the 2016 title, much was made of Dominika Cibulkova being shorter than most tennis players, at five feet three inches. "This is something I've been dealing with since I started to play tennis. I think I've always been brave with my height. Maybe stubborn as well as brave," she said. "I just really wanted to play tennis. No one could tell me something different." 

J is for Jelena Ostapenko. 
Making her debut at last year's WTA Finals, the 20-year-old lost one of her round-robin matches to an opponent 17 years older: 37-year-old Venus Williams. 

K is for Karolina Pliskova. 
One of the highlights of last year's tournament was the "crazy" tiebreak between Pliskova and Wozniacki in the opening set of their semifinal. Leading 6-1, Wozniacki was in command, but couldn't convert any of her five set-points. Then Pliskova had three set-points of her own, but couldn't finish off the tiebreak either. Wozniacki eventually took the breaker 11-9, and went on to win the match in straight sets. "That was a little bit crazy. It was kind of a rollercoaster," said Wozniacki.   

L is for longest gap. 
Eight years passed between Venus Williams' qualification in 2009 and her appearance in last year's WTA Finals. That equalled the record set by Jennifer Capriati, who after qualifying in 1992 didn't then reappear at the tournament until 2000. 

M is for Mirza. 
Showing her all-round class on the doubles court, and in particular her ability to play with different partners, Sania Mirza won the 2014 title with Cara Black and the 2015 tournament with Martina Hingis. 

N is for ninth. 
Singapore is the ninth different city to host the WTA Finals, since the inaugural tournament in Boca Raton in 1972. 

O is for overcoming early setbacks. 
In both 2015 and 2016, the eventual singles champion won only one match from her three appearances in the group stages, but still survived into the knockout rounds. That was Radwanska's route to the 2015 title, and also how Cibulkova won the tournament in 2016.

P is for professor
One of Radwanska's nicknames is 'The Professor', on account of her clever game, which she used to great effect to win the 2015 title. 

Q is for qualification. 
In 2015, the year she reached the semifinals in Singapore, Maria Sharapova was playing at the WTA finals for the eighth time. Among active players, only Serena Williams has played the season-ending tournament more times - her appearance in 2014 was her ninth. Like Sharapova, Radwanska has also played the tournament eight times. 

R is for retirement.
Hingis' last professional match was in the semifinals of last season's WTA Finals, when she and doubles partner Chan Yung-Jan lost to Babos and Hlavackova. 

S is for Shenzhen. 
This is the last year that the WTA Finals will be played in Singapore. From 2019, the tennis elite will gather in the Chinese city of Shenzhen.  

T is for team player. 
While Garbiñe Muguruza is best known as a singles player, her only appearance in the final of the tournament came in the doubles competition. In 2015, she and fellow Spaniard Carla Suarez Navarro were the runners-up to Mirza and Hingis. 

U is for Ukraine. 
Elina Svitolina was a debutante at last year's WTA Finals - she was also the first woman from the Ukraine to qualify for the tournament. 

V is for variety. 
There have been four different singles champions in Singapore, and four different finalists, in Singapore. In 2014, Williams defeated Halep in the final, while in 2015 Radwanska overcame Kvitova. To win the 2016 title, Cibulkova beat Angelique Kerber while Wozniacki fended off Venus Williams to take the season finale last year. 

W is for winning streak. 
Serena Williams' winning streak of 16 matches at the tournament ended in 2014. That was the second longest winning run in the history of the WTA Finals, behind only the 21 matches in a row won by Martina Navratilova. 

X is for extraordinary. 
The last four years that Serena Williams has played the WTA Finals - in 2009 in Qatar, in 2012 and 2013 in Istanbul, and in 2014 in Singapore - she has come away with the title. 

Y is for youth. 
Jelena Ostapenko, who was 20 years old when she competed in Singapore last year, was the youngest competitor since Wozniacki in 2010. She was also the first Latvian to qualify for the WTA Finals. 

Z is for Zimbabwe. 
In the 2014 final, Zimbabwean Cara Black, playing alongside Indian Sania Mirza, gave up just one game against Peng Shuai and Hsieh Su-Wei.