Serena Williams of the USA (Photo:AP)
Williams had six double-faults and won only 51 percent of points on her first serve. But Li also struggled on serve - there were eight breaks in the first set and three more in the second.
Williams closed it out on her third match point.
She has won 28 of her past 29 matches and captured Wimbledon, U.S. Open and Olympic titles this year. The highest ranking she can finish at the end of the year is No. 2 because of injuries and missed tournaments.
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