If Serena turns up in Istanbul in reasonable form, she is the best player in the world - and has been for years. She's the red-hot favourite if she's anywhere near 100 per cent.
There is a huge gulf now between Serena and Azarenka and the rest, and it is expanding. Sharapova would be third on the list, but Azarenka has got her number - while Serena has definitely got Azarenka's number. The other day, when her rivalry with Serena came up, she said: "There's no rivalry!" And there isn't - she can't beat her!
So Serena would go in, as long as she's not too undercooked, as the favourite. Azarenka is now comfortably better than any other woman around her and Sharapova is easily the third-best player around. If she's fit, the tournament is Serena's; if not, it's Azarenka's.
In an Olympics year I'd have the World Tour Championships almost level with the Olympics - maybe a fraction behind. I think the players would agree: huge money, huge bonus points.
And Serena, who has won two Grand Slams and the Olympic singles this year, relishes the big stage - the chance to show that although she might not be number one, she's the best player in the world. To ram it down people's throats just how good she is.
There have been blips along the way, sure, but they are always more to do with Serena than anybody else. She's just far too good for anybody else and she's won two WTA Tour Championships before.
I don't think that Sharapova's past success in the 2004 WTA Tour finals, where she beat Serena in the final, gives her an edge on Azarenka. Maria has done really well and she's elevated herself out of the pack, but Azarenka has even more - and beat her fairly convincingly in Australia when it mattered.
Azarenka moves just too well for Sharapova and when she's really got her game face on, as she has at the moment, she's hard to stop. I'd be mightily surprised if she doesn't win this tournament in Beijing this week, with Serena is absent with flu. If she goes in with form and confidence, I don't see her being picked off by anybody other than Serena.
If any of the big guns fall away, are ill or have off-days, the likes of Angelique Kerber, Radwanska and Li Na come into play.
Radwanska hasn't really kicked on as I thought she might after reaching her first Grand Slam singles final. Plus she's got a lot of taping around her shoulder at the moment, indicating that she has a problem there. If the big-hitters have an off-day, Radwanska gets in there, as we saw at Wimbledon; but if they are playing well, I just don't think she stacks up. I think she'll do okay, but I don't see her beating the top three in big matches.
You never know with Li. She got a lot of stick last year because she didn't play well in this tournament, and it means a lot to the Chinese. But she's in a much better place this year and I think she's determined that it is going to go right - at the end of last year it was a very long year and she started to fall away a bit. It's very important that she does well in China, where a lot of her sponsorship is built up, and still needing points for the WTA Champs could make the difference for her form. She's doing okay at the moment, but I think she might do very well this week: if she consistently plays well, she can be a threat. And if she gets to the WTA Championships she's got a big game - but you just can't pick it with her.
Sara Errani's season is beginning to flag now. It's been an amazing year and she's elevated herself out of all recognition compared with what she was before, but it's starting to take its toll now. When she gets into the big matches with the consistently big-hitters, she doesn't really stack up.
It was monumental what happened, her getting to the final at Roland Garros, but I think that's where it ends. At that level, I don't see her repeating that. She'll knock off a good few players here and there, but I would really fear for her at the end-of-tour championships. She could easily go home empty, but without a win.
Petra Kvitova has also been knocked out in Beijing this week. She has always looked a little vulnerable - it's always all-or-nothing with her - and at the moment it's nothing. Caroline Wozniacki and Marion Bartoli can still sneak in, but they need to be getting to the final in China then have a couple more big tournaments to stand a chance. However, the way it stands at the moment, as I said, it looks like a Sharapova-Azarenka final.
Laura Robson fell at the first hurdle in China, but we shouldn't worry about it not happening every week for her. Can she reach the World Tour Champs in the future? I'm not sure - I see her as a potential top-20 player, but top eight is a big bridge to cross from there.
Obviously she's still only 18 and if you look at those going for the Tour Championships, there are no teenagers in there - three or four years' time is when we could be asking the question of whether she will make the end-of-season championships.