US Open Day 9 - four matches to watch

1 / 2
Defending champ: Rafael Nadal celebrates his match point against Nikoloz Basilashvili in the fourth round of the US Open

Four matches to watch on Tuesday, the ninth day of the US Open (x denotes seeding):

Rafael Nadal (ESP x1) v Dominic Thiem (AUT x9)

World number one Nadal will seek to continue his US Open title defense, taking on Thiem for a place in the semi-finals.

Ninth-seeded Austrian Thiem is in his fourth Grand Slam quarter-final, but his first outside Roland Garros -- where he was soundly beaten in the final by Nadal three months ago.

Nadal has beaten Thiem three times at the French Open and seven times in 10 meetings -- all on clay.

Thiem admitted he was hoping the hard courts of Flushing Meadows would prove "more comfortable" but he'll still face an uphill battle against the 17-time Grand Slam champion whose stellar 2018 campaign also includes Masters titles in Monte Carlo, Rome and Toronto.

Serena Williams (USA x17) v Karolina Pliskova (CZE x8)

Eighth-seeded Czech Pliskova insists she has the game to challenge 23-time Grand Slam champion Williams as they battle for a semi-final berth.

Pliskova, the three-time WTA tour ace leader who briefly ascended to world number one last year, has surprised Williams before at the US Open, beating her to reach her first Grand Slam final in 2016.

"I know she has a big game, but I have a big game too," said Pliskova.

Williams, a six-time champion who missed last year's US Open as she gave birth to her daughter Olympia, is on a mission, seeking a title that would see her match Australian Margaret Court's all-time record of 24 major titles and let her avoid her first year since 2011 without a Grand Slam crown.

Juan Martin del Potro (ARG x3) v John Isner (USA x11)

Giants collide when third-seeded del Potro -- who claimed his lone Grand Slam title at the 2009 US Open -- takes on Isner -- the last American main remaining in the draw -- for a semi-final berth.

Del Potro, the 6-foot-6 (1.98m) Tower of Tandil knows it will be a battle against 6-foot-10 (2.08m) Isner, who has blasted a tournament-leading 112 aces.

Both players have reached Grand Slam semi-finals this year, del Potro at Roland Garros where he cruised past Isner in the fourth round and Isner at Wimbledon.

Which will make it to another Grand Slam final four on Tuesday?

Sloane Stephens (USA x3) v Anastasija Sevastova (LAT x19)

Defending champion Stephens finds a familiar foe across the net in Sevastova, beaten by the American at the same stage last year as Stephens roared to the title as an unseeded player.

Since her maiden Grand Slam triumph, Stephens has reached a second major final at the French Open.

She has reached the quarter-finals with the loss of just one set. Sevastova has gone three sets in three of her first four matches, including a roller-coaster fourth-round victory over seventh-seeded Elina Svitolina.