Serena Williams saw off a significant threat to her Grand Slam hopes with victory over 19th seed Madison Keys at the US Open.Williams, the world number one, beat her fellow American 6-3 6-3 to set up a quarter-final against her sister Venus.Serena, 33, is now three wins from competing the first calendar Grand Slam since Steffi Graf in 1988.Novak Djokovic overcame an in-form Roberto Bautista Agut, subduing the Spanish 23rd seed 6-3 4-6 6-4 6-3.The top seed, who has already won the Australian Open and Wimbledon this season, will face Bautista Agut`s compatriot Feliciano Lopez in the next round.Men`s defending champion Marin Cilic and 19th seed Jo-Wilfried Tsonga both won and will meet in the last eight.