Agnieszka Radwanska in action against Petra Kvitova.
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ISTANBUL (AP) -- Agnieszka Radwanska defeated titleholder Petra Kvitova 6-3, 6-2 to start round-robin play at the WTA Championships on Tuesday.
Kvitova entered the match with a 25-match winning streak indoors but she was in trouble early, losing the first three games. Although she recovered to tie at 3-all, Radwanska broke serve again and served out the set.
Kvitova dropped her first two service games of the second, and broke back once. But the former Wimbledon champion was unable to threaten the steadier Radwanska.
It marked only Kvitova's second loss - and her first in four matches against Radwanska - in 28 indoor matches over two years. She was undefeated in winning the title last year.