World number one Simona Halep's withdrawal from the WTA Finals through injury has left this year's tournament in Singapore wide open as eight women bid to prove they are the best around.

Three of this year's grand slam champions - Caroline Wozniacki, Angelique Kerber and Naomi Osaka - are part of the 'elite eight', while Petra Kvitova is seeking to win the competition again seven years after she first did.

Kerber is in a group with three tournament debutants - Osaka, Sloane Stephens and Kiki Bertens, while previous champions Wozniacki and Kvitova are in the other round-robin pool alongside Elina Svitolina and Karolina Pliskova.

We run the rule over those involved ahead of the competition beginning on Sunday.

1. Angelique Kerber

The top seed following Halep's withdrawal, Kerber missed the tournament in 2017 due to a late-season slump but returned to form in 2018 and captured her third grand-slam title at Wimbledon.

World ranking: 3rd

Titles in 2018: Two (Sydney International, Wimbledon)

Last five tournaments (most recent first): China Open - last 16, Wuhan Open - last 16, US Open - last 32, Cincinnati Masters - last 16, Rogers Cup - last 32

WTA Finals history: 2016 - runner-up, 2015 - first round, 2013 - first round, 2012 - first round

2. Caroline Wozniacki

Having started the year by clinching her first grand slam title in Melbourne, the defending WTA Finals champion arrives in fine form having also won her most recent tournament in China.

World ranking: 2nd

Titles in 2018: Three (Australian Open, Aegon International, China Open)

Last five tournaments: China Open - winner, Wuhan Open - last 16, Tokyo Open - last 16, US Open - last 64, Cincinnati Masters - last 32

WTA Finals history: 2017 - winner, 2014 - semi-finals, 2011 - first round, 2010 - runner-up, 2009 - semi-final

3. Naomi Osaka

Osaka, who turned 21 this week, has enjoyed a breakthrough campaign in 2018, winning her first two WTA titles. Both were hugely significant triumphs as she followed a stunning victory at Indian Wells by claiming US Open glory, beating her idol Serena Williams in the final at Flushing Meadows.

World ranking: 4th

Titles in 2018: Two (Indian Wells Masters, US Open)

Last five tournaments: China Open - semi-finals, Tokyo Open - final, US Open - winner, Cincinnati Masters - last 64, Rogers Cup - last 64

WTA Finals history: No previous appearances

4. Petra Kvitova

No woman has won more WTA titles than Kvitova in 2018 - could she make it half-a-dozen victories by following up her win at the Finals in 2011 with another success?

World ranking: 7th

Titles in 2018: Five (St Petersburg Ladies Trophy, Qatar Open, Prague Open, Madrid Open, Birmingham Classic)

Last five tournaments: China Open - last 64, Wuhan Open - last 16, US Open - last 32, Connecticut Open - quarter-finals, Cincinnati Masters - semi-finals

WTA Finals history: 2015 - final, 2014 - first round, 2013 - semi-finals, 2012 - first round, 2011 - winner

5. Sloane Stephens

The US Open champion in 2017 had a tough time on the Asian swing but has done enough to make her maiden WTA Finals, having reached the French Open final earlier in the year.

World ranking: 8th

Titles in 2018: One (Miami Open)

Last five tournaments: Kremlin Cup - last 16, China Open - last 16, Wuhan Open - last 64, Tokyo Open - last 32, US Open - quarter-finals

WTA Finals history: No previous appearances

6. Elina Svitolina

Svitolina made the WTA Finals in 2017 - recording a straight-sets win over Halep at the competition - and three tournament victories in 2018 sealed her spot again.

World ranking: 6th

Titles in 2018: Three (Brisbane International, Dubai Tennis Championships, Internazionali d'Italia)

Last five tournaments: Hong Kong Tennis Open - quarter-finals, China Open - last 64, Wuhan Open - last 32, US Open - last 16, Cincinnati Masters - quarter-finals

WTA Finals history: 2017 - first round

7. Karolina Pliskova

The Czech is into the WTA Finals for a third straight year, having won two titles and reached two grand slam quarter-finals.

World ranking: 5th

Titles in 2018: Two (Stuttgart Open, Tokyo Open)

Last five tournaments: Kremlin Cup - last 16, Tianjin Open - final, China Open - last 16, Wuhan Open - last 32, Tokyo Open - winner, US Open - quarter-finals

WTA Finals history: 2017 - semi-finals, 2016 - first round

8. Kiki Bertens

Bertens was in doubles action at the WTA Finals in 2017 but Halep's withdrawal has ensured she is one of the 'elite eight' in the singles.

World ranking: 10th

Titles in 2018: Three (Charleston Open, Cincinnati Masters, Korea Open)

Last five tournaments: Kremlin Cup - last 16, Linz Open - last 16, China Open - last 16, Wuhan Open - last 32, Korea Open - winner

WTA Finals history: No previous appearances