World number one Novak Djokovic's preparations for the US Open suffered another blow after a straight-sets defeat to Tommy Robredo in the third round of the Western & Southern Open.
Djokovic, who looked to be back to his imperious best in winning Wimbledon last month, has struggled with the transition onto the American hard courts and with the final major of the year a little over two weeks away, doubts will be raised as to whether the 27-year-old can make a credible challenge.
He was thumped by Jo-Wilfried Tsonga at the Rogers Cup last week and any thoughts that may have been an aberration vanished following a 7-6 (8/6) 7-5 defeat to 16th-seeded Spaniard Robredo in one hour and 44 minutes.
"Just many, many, many things are not clicking these two weeks on hard courts," said Djokovic on the ATP Tour website.
"It's unfortunate, but it's more than obvious I'm not playing even close to what I'm supposed to play. I have to keep on working and trying to get better for the US Open."
The ongoing tournament in Cincinnati is the only Masters title that Djokovic has not won, although he has been a finalist on four occasions.
The opportunity, therefore, to complete a career 'Golden Masters' has been put on hold for another year following Thursday's defeat.
Djokovic was made to toil in a three-set victory over Gilles Simon 24 hours earlier and looked uncomfortable throughout, making 30 unforced errors before limply conceding his serve in the final game to give 32-year-old Robredo victory.
It was a better day for the other top-10 seeds in action early on Thursday, with Roger Federer, Stan Wawrinka, Milos Raonic and Andy Murray all sealing their passage into the quarter-finals.
However, all four had to come through three sets to advance, with Federer beating Gael Monfils 6-4 4-6 6-3 while third seed and fellow Swiss Wawrinka won 13 of the final 14 games in a 3-6 6-0 6-1 victory over Marin Cilic.
Fifth seed Raonic lost a first-set tie-break against world number 55 Steve Johnson but managed to avoid the upset in a 6-7 (7/9) 6-3 7-6 (7/4) success, while Murray, seeded eighth, saw off home favourite John Isner 6-7 (3/7) 6-4 7-6 (7/2).
Fifth seed David Ferrer brushed aside Mikhail Youzhny 7-5 6-0 and will face fellow Spaniard Robredo, Fabio Fognini defeated Yen-Hsun Lu 3-6 6-3 6-3 to set up a clash with Raonic while Julien Benneteau will meet Wawrinka after a 7-5 6-1 triumph over Jerzy Janowicz.