- Muguruza, Halep the top two at WTA Finals
- Three players making debuts at year-end championships
- Venus Williams, Caroline Wozniacki return
- Last year's winner Dominika Cibulkova didn't qualify
(CNN)An eventful season on the women's tennis tour? You could say that.
Serena Williams won the Australian Open while pregnant, Petra Kvitova came back after surviving a knife attack in her home and Maria Sharapova returned following her drug ban. Five different players, meanwhile, held down the No. 1 ranking.
The singles season concludes with the WTA Finals in Singapore -- which starts Sunday -- and seven of the eight players in the draw have a chance to end the campaign at No. 1.
Here's a look at field, which features players from eight different nations.
Simona Halep: The current No. 1 became the first Romanian woman to ever hold down top spot.
After spurning opportunities to get to No. 1 in the spring and summer, Halep overtook Garbine Muguruza by reaching the final in Beijing earlier this month.
But if that was a highlight for the 26-year-old, she has suffered some very tough defeats in 2017.
Halep relinquished a set and 3-0 lead in the French Open final -- clay is probably her best surface -- to Jelena Ostapenko, fell to Johanna Konta in a quarterfinal thriller at Wimbledon and lost to Sharapova in the first round at the US Open in yet another superb tussle.
Winning in Singapore, then, would be especially sweet for Halep.
Garbine Muguruza: The Spaniard clearly loves the big stage.
By the time Muguruza won Wimbledon, she had amassed four career titles -- and two of them were grand slams.
The 24-year-old became the first Spanish woman to triumph at the All England Club since Conchita Martinez -- one of her coaches at Wimbledon -- in 1994 and the first player to defeat both Williams sisters in a grand slam final.
It came weeks after a raucous crowd at the French Open left her in tears. The Venezuelan-born Muguruza had struggled in the aftermath of claiming the French Open in 2016 but ensured there would be no slump after her fortnight at Wimbledon as she was crowned champion in Cincinnati in August.
Karolina Pliskova: Only three players in the field this year took part last year -- Halep, Muguruza and Pliskova.
The first half of the season for the tour's ace leader was better than the second, with the Czech winning two titles and making the semifinals during the "Sunshine Double" of Indian Wells and Miami.
Then there was a surprising semifinal showing at the French Open -- clay isn't the 25-year-old's preferred surface.
But as one of the favorites at the Australian Open and perhaps the favorite at Wimbledon, Pliskova exited in the quarterfinals and second round, respectively, prior to a quarterfinal defeat at the US Open to CoCo Vandeweghe.
"I was not playing my best tennis this tournament," said Pliskova in New York.
Elina Svitolina: The Ukrainian's year-end ranking has improved every year since turning pro -- and that will continue no matter what happens at the WTA Finals.
Making her debut at the WTA Finals along with Ostapenko and Caroline Garcia -- Svitolina is perfect in finals this year, going 5-0. Her three biggest successes came in Dubai, Rome and Toronto.
But like Halep, the counter-puncher was dealt some difficult losses: Surrendering a set and 5-1 lead to Halep in the French Open quarterfinals and 4-2 third-set advantage to Madison Keys in the fourth round at the US Open.
Worth noting: The 23-year-old pulled out of her second-round match in Hong Kong last week with a leg injury.
Wozniacki, Venus return
Caroline Wozniacki: After an injury plagued 2016, a healthy Wozniacki has been one of the tour's most consistent performers.
When Wozniacki repeated as champion in Tokyo in September, it ensured that the Dane won a title for a 10th straight season.
There must have been some relief for Wozniacki, since the 27-year-old lost all six of her previous finals this year.
The former No. 1 is making her fifth appearance at the year-end championships, reaching the final in 2010 in Doha before falling to Kim Clijsters.
Like Svitolina, Wozniacki was forced to give her opponent a walkover in Hong Kong, hindered by an elbow injury.
Venus Williams: Williams is the lone former champion in the field, winning in 2008, then losing to Serena in the 2009 final.
That appearance eight years ago was her last trip to the year-end championships. Venus -- now 37 -- turned back the clock, reaching the Australian Open final and Wimbledon final, along with the US Open semis.
No one on the women's tour won more grand slam matches than the American. And if she had taken one of her two set points in the first set against Muguruza at Wimbledon, Venus just might have captured her first major since 2008.
Jelena Ostapenko: Ostapenko was a high-profile junior, winning at Wimbledon as a 17-year-old in 2014.
But who saw her title at the French Open coming? The Latvian lost in the first round of the juniors in 2014, first round of qualifying in 2015 and in the first round proper in 2016. And she'd never won a top-level title before.
Ostapenko, however, put it all together at Roland Garros, capping her run with the comeback victory over Halep.
Ostapenko had a fruitful Asian swing, too, winning in Seoul and landing in the semifinals in Beijing and Wuhan. If she goes to a third set, look out: Ostapenko is 22-8 in deciders this year, including winning 20 of her last 24.
Caroline Garcia: Who's the most in-form player heading into Singapore? Garcia, whom Andy Murray spoke highly of six years ago.
An outsider to qualify for the WTA Finals following the US Open -- the Frenchwoman struggled with a back injury early in the year -- Garcia made a late charge, becoming the first woman to win Wuhan and Beijing in the same season. She thus passed Konta, who was edged out for the second straight season.