Serena Williams welcomed a tight battle against her sister Venus with the US Open fast approaching.
In the 31st meeting between the sisters, Serena Williams recovered for a 3-6 6-3 6-4 victory against Venus on Thursday.
Barbora Krejcikova put up a fight in Prague, but Simona Halep eventually passed the test to reach the last eight.
Novak Djokovic has confirmed he will travel to the United States to play at the Western and Southern Open and US Open.
Serena Williams will meet sister Venus for a 31st time after winning at the Top Seed Open.
Serena Williams was five points from defeat against Bernarda Pera but found some form to claim a three-set victory in Lexington.