Williams warns Istanbul rivals she can still get better ISTANBUL (Reuters) - Serena Williams aims to claim her 11th tournament trophy to round off one of the best years of her career, and warned her opponents at this week's WTA season-ending championships that she can still improve. It's definitely one of the best years I have had... I'm still looking to improve and... I have a lot of room for improvement," Williams told reporters on Monday, the day before the eight-woman tournament begins. The American world number one has won more matches and more titles than in any other year of her career, losing four times in 73 games in 14 events. "I'm so excited for next year just to take my game to a new level," added Williams, who won the French and U.S. Opens this year to take her grand-slam total to 17. It's just been a great opportunity for me to continue to play, so I never thought I would have one of my best years this year but I always just try to keep doing better. Defending champion Williams has been drawn with Agnieszka Radwanska, Petra Kvitova and Angelique Kerber in the red group at Istanbul's Sinan Erdem dome while second seed Victoria Azarenka, Li Na, Sara Errani and Jelena Jankovic are in the white group. World number three Maria Sharapova will miss the championship because of a shoulder injury. Williams said she expected a good match when she plays Kvitova, a dangerous opponent on a fast indoor surface, although the Czech player has never beaten her. Second seed Azarenka, who has beaten Williams twice in 2013, is her most likely title rival, and the Belarussian said she was looking forward to taking on the American again. Sometimes when you feel you don't want to go to the gym or the tennis court and fight, somebody is above you that you want to reach and grab that spot. Definitely that's a big motivation," Azarenka said. If Azarenka wins, it would be her first title at the end-of-season event after she lost in the final to Kvitova two years ago in Istanbul which is staging the event for the final time before it moves to Singapore.