Serena still looking to improve
Serena Williams will be competing in the WTA Championships in Istanbul this week (AP)
Serena Williams warned her rivals she is ready to take her game to a new level as she prepared to begin her campaign for an 11th title of an extraordinary year.
At 32, the American is more dominant than at any stage of her career, having won more trophies and more matches at this point in a season than ever before.
It would be a big surprise if she did not retain her title at this week's end-of-season WTA Championships in Istanbul, which features the best eight players of the year.
Williams has shown a consistency of play but most importantly of desire since linking up with coach Patrick Mouratoglou that had previously been lacking, and she has no intention of cutting the rest of the women's game any slack.
She said: "W hen I was 20 or 18 I never thought I'd be playing at this stage, and I have had a lot of time to think about it.
"The sport has just been growing so great. It's been a great opportunity for me to continue to play. I never thought I would have one of my best years this year, but I always just try to keep doing better.
"What I think is great about it is I'm still looking to improve, and what I learned most about this year is I have a lot of room for improvement. And talking with my coach over it, I'm so excited for next year just to take my game to a new level."
Williams began her winning run at Wimbledon last year and has not looked back, collecting 15 singles titles - including four grand slams and an Olympic gold medal - and losing only five matches.
One of those losses came against Victoria Azarenka in the final in Doha in February, but it was a special week nonetheless.
A quarter-final victory over Petra Kvitova had guaranteed Williams would reclaim the world number one ranking only two years after she came close to losing her life from a pulmonary embolism.
"I have been through so much the past few years and I thought if anything it was a great story not only for me but for everyone just to realise that it doesn't matter what happens to you as a person, that if you just keep going and you never give up, you can have an opportunity to survive," Williams said.
"For me, I feel all those overwhelming emotions, and then to be number one again, it was really a great feeling and something I never expected."
The round-robin section of the tournament begins on Tuesday, with Williams drawn in the red group alongside Agnieszka Radwanska, Petra Kvitova and Angelique Kerber.
Considering she has a 13-1 record against the other players in her group, the remaining trio are likely to be fighting for one spot in the semi-finals.
German Kerber, who is the only one of the three to have beaten Williams, is her first opponent, with Radwanska playing Kvitova in the final match of the first day on Tuesday evening.
The white group features Li Na, Jelena Jankovic, Victoria Azarenka and Sara Errani, and it is the latter two who open proceedings.
World number two Azarenka is the only player to have beaten Williams more than once this year and her most likely title rival.
For the second year in a row, the two contested an enthralling US Open final, with Azarenka recovering from 4-1 down in the second set to level the match, but again it was Williams who came out on top.
The rivalry is an important one for both players and for the game, and Azarenka said: " I think it's been growing.
"I think it's been becoming more and more interesting, and we had a lot of very good battles. I'm honoured to be a part of that. That's what makes me every time work harder to get to the next level, and I'm excited about that."
Jankovic has had the most success against Williams of all the players in the field, with four wins from 11 meetings, although none since 2010, while, like Kvitova and Radwanska, Errani has not won a single match.
The Italian suffered the humiliation of winning only one game against Williams in the semi-finals of the French Open, but she insists there is reason for optimism for her and everyone else.
"Of course Serena is very strong at the moment," Errani said. "She's very consistent, so it's not easy, but everybody can have a bad day, so why not?
"Everybody wants to go there and win and try their best. Maybe if she's not 100 per cent or the others are playing much better, I think also she can lose. It's not impossible. It's difficult but not impossible."