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.

Men's defending champion Marin Cilic and 19th seed Jo-Wilfried Tsonga both won and will meet in the last eight.

Top seed Novak Djokovic plays 24th seed Roberto Bautista Agut later on Monday.

Williams knew she faced a serious threat in the powerful 19-year-old Keys, and the six-time champion produced a clinical performance.

She had looked nervous and on edge throughout the first week, with her normally impregnable serve vulnerable - the most dominant shot in tennis was broken five times in three matches.

Williams hit the practice court hard after her third-round match, and she would not offer up a single break point against Keys.

"I served much better today," Williams said afterwards. "I'm so proud I was able to serve better.

"Obviously I had to - I knew I wouldn't have too many chances to break."

In a match dominated by short rallies of fewer than four strokes, Keys cracked first with two double faults in a row to give up the break in game eight.

Williams duly served out and kept the pressure up in the second set, missing a host of break points before converting the seventh to lead 4-3.

She did not require any help, but a net cord handed Williams match point and a Keys double fault brought an end after 68 minutes.

The next player in Serena's path is her 35-year-old sister Venus, who beat Estonia's Annett Kontaveit 6-2 6-1.

(BBC)