Serena Williams' 19-match winning streak ended with defeat against Angelique Kerber in the Western & Southern Open quarter-finals.
The American, who had not lost a set since she beat Agnieszka Radwanska to win the Wimbledon title, was defeated 6-4 6-4 by her German opponent.
Elder sister Venus did not slip up, reaching her first semi-final of the season with a 6-2 6-7 (2/7) 6-4 victory over Sam Stosur.
Anastasia Pavlyuchenkova bowed out after being forced to play two matches in a day.
After wrapping up a 6-4 6-4 victory in her third round contest against Caroline Wozniacki - a match held over because of poor weather on Thursday - she lost 6-3 7-6 (7/4) to fourth-seeded Petra Kvitova, who now meets Kerber for a place in the final.
However, two matches in a day did not pose problems for Li Na.
After beating Swedish qualifier Johanna Larsson 6-2 6-2 to make sure of a place in the last eight, she saw off top seed Radwanska 6-1 6-1 to book a semi-final against Venus Williams.
Update:
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