This year’s tennis Grand Slams — the four most prestigious tournaments — have all been played. But the biggest payday still awaits at the Women’s Tennis Association Finals, which start Sunday in China. This is the last event of the season. Canada’s Bianca Andreescu is there. Here’s what else you need to know about it: Who else is playing? There’s both a singles and a doubles tournament at the WTA Finals, and Canada’s Gabriela Dabrowski is competing in the doubles with her Chinese teammate Xu Yifan. But we’re just going to focus on the singles here. As at any big tennis event, that’s the tournament people really care about. Only the top eight women in the world are invited to the singles tournament. From highest seed to lowest, they are: Australia’s Ash Barty, Czech Republic’s Karolina Pliskova, Japan’s Naomi Osaka, Andreescu, Romania’s Simona Halep, Czech Republic’s Petra Kvitova, Switzerland’s Belinda Bencic and defending champ Elina Svitolina of Ukraine. Serena Williams did not qualify. How does it work? Unlike most tennis tournaments, which are single elimination, the Finals follow a round-robin format. The players are divided into two groups. Andreescu is in the purple group with the No. 2 seed Pliskova, No. 5…