NEW YORK • Serena Williams and Marin Cilic survived third-round scares on Friday to keep their US Open title defences on track.

A day of riveting high drama began in the early afternoon heat with the ninth-seeded Cilic needing more than four hours to put away Kazakhstan's Mikhail Kukushkin 6-7 (7-5), 7-6 (7-1), 6-3, 6-7 (7-3), 6-1.

There was even more suspense to follow as Williams came back to prevail 3-6, 7-5, 6-0 over Bethanie Mattek-Sands to keep alive her quest for a calendar Grand Slam.

Williams, who dropped just nine sets, including seven first-set losses, in her charge to the Australian, French and Wimbledon titles this year, was tested at Flushing Meadows by her fellow American.

"I'm not trying to live on the edge," the 33-year-old Williams said. "I'm playing players that come out really strong. I just think Bethanie played really well."

The 21-time Grand Slam singles winner, aiming to join Maureen Connolly (1953), Margaret Court (1970) and Steffi Graf (1988) as the only women to bag the four Grand Slams in a single season, next faces American Madison Keys.

Her path to the final was eased by sister Venus who ousted the top-rated rival on their side of the draw, defeating 12th-seeded Swiss teen Belinda Bencic 6-3, 6-4.

The latter inflicted the most recent defeat on Serena last month in a Toronto semi-final but fell to 0-4 against Venus, who fired 31 winners against 15 unforced errors.

"I just wanted to be aggressive," Venus said.

"Keeping the errors down really helped me close out the match."

Venus, seeded 23rd, reached a fourth-round match against 152nd-ranked Estonian teen qualifier Anett Kontaveit and could face Serena after that.

"I hope we both get to the quarter-finals," Venus said.

Earlier, Croatian ninth seed Cilic barely escaped making the earliest exit of any defending champion since Andre Agassi in 1999 before outlasting 56th-ranked Kukushkin.

"It wasn't easy but I stayed together and got it done," Cilic said.

He next meets French 27th seed Jeremy Chardy who ousted Spanish seventh seed David Ferrer 7-6 (8-6), 4-6, 6-3, 6-1.

After a week of punishing heat that saw a spate of player retirements, cooler temperatures returned to Flushing Meadows as top seed Novak Djokovic eased to a third-round 6-3, 7-5, 7-5 victory over Italian Andreas Seppi.

It has been a trouble-free visit to New York for Djokovic, the top seeded Serb not dropping a set while losing just 10 games going into his match against Seppi.

But the 25th-ranked Italian made Djokovic work for his spot in the round of 16, with the world No. 1 needing 21/2 hours to secure victory.

REUTERS, AGENCE FRANCE-PRESSE