By Frank Pingue
NEW YORK (Reuters) - Serena Williams has been unstoppable in New York but faces her toughest test yet when she battles Karolina Pliskova on Tuesday in the quarter-finals at the U.S. Open.
Top-seeded Spaniard Rafael Nadal and world number three Sloane Stephens are also looking to keep alive their hopes of successfully defending their titles in their matches against Austrian Dominic Thiem and Latvian Anastasija Sevastova.
Williams, who is seeking her first Grand Slam title since becoming a mother last September, has been in top form, facing only nine break points through four matches, with her only real blip a second-set stumble during the fourth round.
But eighth seed Pliskova, a former world number one and U.S. Open finalist in 2016, presents a tough task for Williams and is the last player to beat her in New York, having got the better of the six-times champion in the semi-finals two years ago.
Williams, who is three wins shy of a record-equalling 24th Grand Slam title, won her only other match-up with Pliskova - at Stanford in 2014 - and will surely have the home crowd on her side during a match that will kick off the evening session.
Stephens, who is looking to become the first woman to successfully defend a Grand Slam title since Williams triumphed at Wimbledon in 2015 and 2016, will start the day's action inside Arthur Ashe Stadium against 19th-seeded Sevastova.
The 25-year-old American, who struggled mightily after her triumph last year, has since righted the ship and will be heavily favoured to deny Sevastova from becoming the first Latvian woman to reach the semi-finals of the U.S. Open.
Argentine Juan Martin del Potro, the 2009 champion, will cap the day session when he faces 11th-seeded John Isner in a heavyweight slugfest of towering big-servers.
Del Potro, seeded third, has won 81 percent of his first serves en route to the last eight while Isner, looking to become the first American man to reach the U.S. Open semi-finals since Andy Roddick in 2006, has won 85 percent of his own.
French Open champion Nadal has been tested in his run to the quarters having been taken to four sets in each of his last two matches and could be in for another battle against Thiem.
The Austrian is brimming with confidence having reached the first Grand Slam final of his career this year at Roland Garros and showing impressive form in New York where he has dispatched last year's runner-up Kevin Anderson in the fourth round.
(Reporting by Frank Pingue; Editing by Alison Williams)