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The fourth seed, who is the only Chinese player to win a Grand Slam having triumphed at the 2011 French Open, clinched the first five games to storm into a commanding lead in the opening set.
She broke her Italian opponent, ranked number 29 in the world, at the first three attempts, saved two break points of her own and recorded a hold to love in 20 minutes of one-way traffic.
Pennetta got on the scoreboard by winning the next two games, but Li went on to wrap the first set up 6-2 and then repeated her rapid start by claiming four straight games at the start of the second.
Again Pennetta ensured that she did not fall to a bagel set defeat as the pair exchanged games until Li served out for a 6-2 6-2 victory, setting up a last-four clash with either Eugenie Bouchard or Ana Ivanovic.