Serena Williams races to win over Kerber at WTA Championships

Serena Williams powered past Angelique Kerber in her opening match at the WTA Championships in Istanbul.

The world number one broke the German's serve at the first opportunity in each set on the way to a 6-3 6-1 success.

Second seed Victoria Azarenka was made to work hard for a 7-6 (7-4) 6-2 victory over Italian world number seven Sara Errani.

Poland's Agnieszka Radwanska, seeded third, lost 6-4 6-4 to the Czech Republic's fifth seed Petra Kvitova.

American Williams, already guaranteed to end the season on top of the world rankings, is aiming to become the first player to successfully defend the season-ending title since Justine Henin in 2007 and win a fourth crown overall.

She was simply too strong for world number nine Kerber, conjuring 31 winners, but also mixing her game to win the point on each of the 14 occasions that she ventured to the net.

"I feel really good," said Williams. "She's beaten me before so I was a little nervous and really focused.

"I never usually say I'm happy about a match, but I am now and I'm excited about that.

"It's such a tough tournament. Every match you have to be focused. I'm happy, but I have to get focused again."

Williams's dominance was in contrast to the earlier struggles of Azarenka, who was 5-2 down to Errani in the first set before fighting back to take it on a tie-break in an hour and 28 minutes.

Only when Errani called the trainer for treatment on a calf injury did matters become simpler for the Belarusian, with the two-time Australian Open champion winning the final four games to complete victory.

Meanwhile, British number three Heather Watson claimed only a second win in two months with a hard-fought success over American Christina McHale at the ITF tournament in Poitiers, France.

Watson, who reached a career-high ranking of 39 in February, has struggled since returning from a bout of glandular fever at the French Open in May and has slipped to 133.

Dropping down from the WTA Tour to play one of the bigger events on the second tier, Watson lost the first set and, after fighting back, trailed 4-2 in the decider.

But she won four successive games to come through 2-6 7-5 6-4.