Serena Williams wary of Madison Keys challenge at US Open

Serena Williams has admitted that fourth-round opponent Madison Keys could pose a big threat to her calendar Grand Slam hopes at the US Open.

The world number one and defending champion at Flushing Meadows beat Bethanie Mattek-Sands 3-6 7-5 6-0 this morning to set up another all-American affair.

Keys's route to the fourth round was markedly less dramatic, as the 20-year-old thumped Agnieszka Radwanska 6-3 6-2.

"She has a really big game and she's really good," Williams, who is vying to add to her Australian Open, French Open and Wimbledon titles in 2015, told ESPN.

"She's on a roll right now and she's playing really well and with a lot of confidence."

Williams beat Keys in an Australian Open semi-final earlier this year for what remains the only meeting between the duo.

Should she progress, Williams could face big sister Venus in the quarter-finals, having won 15 of the 26 senior competitive meetings with her sibling.