"I just didn't have a spark, " stated the 23-year-old Russian, who served up eight double faults.
Dementieva served 19 double faults against Japan's Akiko Morigami but still came through 6-3 4-6 6-3.
But, in contrast to last week, the 21-year-old made numerous unforced errors and hit as many double faults as aces.
Hewitt, without a title since his Wimbledon win, took advantage of two double faults to close out the tie-break and the match.
But the fourth seed's serve became more erratic, making four double faults, and Nadal capitalised with two breaks to seal the first set.
The 21-year-old had the chance to win the first set when serving at 5-3 but three double faults handed the Scot a lifeline.
Two double faults were followed by an errant smash, and as Clijsters suddenly discovered her second wind, the American cut a disconsolate figure.
The world number 62 could not have started better as the Croat produced two double faults before being broken in the opening game of the match.
Hewitt appeared shaken, serving two double faults in the next game to lose his serve and give the South African the chance to serve for the match.
The Belgian world number 18 broke her opponent twice en route the first set, but five double faults in the second helped Safarova get back on level terms.
Clijsters, who slumped to number 133 in the world rankings because of her injury problems, overcame five double faults before trumping Asagoe with a host of forehand winners.
Her mood darkened further when Clijsters fashioned an early break in the decider, but at 3-4 the Belgian threw in a couple of double faults of her own before sending a drive volley long.
With a little more composure, 19-year-old Gulbis, bidding to become the first Latvian to reach a Grand Slam semifinal, could have pulled off a famous win, but his 60 unforced errors and seven double faults told a sorry story.
It is the second time in as many tournaments that Dimitrov has cracked when serving for a set against a top-ranked player, having served four double faults when leading world No. 1 Novak Djokovic 5-3 at Indian Wells earlier this month.
Two breaks in a first set that included no double faults or unforced errors would have settled any early nerves, and she eased through the second set in 35 minutes to banish any lasting memories of her defeat in London in June.
Sharapova committed two consecutive double-faults to drop her serve at the start of the decisive set.
Sharapova had 57 unforced errors, including 13 double-faults, and overcame three set points in the second set.
The Serbian made 40 unforced errors and served four double-faults as she lost to Bondarenko for the first time in 10 meetings.
All told, Sharapova benefited from eight double-faults by the No. 29-seeded Vesnina, and erased eight of the nine break points she faced.
The Russian made 30 unforced errors, three double-faults and hit only won clean winner as she lost in 91 minutes to the unseeded 31-year-old.
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Venus struggled with her serve in the first set, making five double-faults, but still won it easily as both players went for their shots.
Seeded third, Azarenka hit 10 double-faults and was broken six times.
Serve was again key with the Thai player hitting four aces and just one double-fault while the German had just one ace and five double-faults.
She overcame eight double-faults, the erroneous ruling and a second-set deficit to beat Zheng Jie 6-1, 7-5 in the third round Saturday at the French Open.
Two aces by Djokovic and two double-faults by Davydenko helped to decide the first set tie-break only for the match to be turned on its head completely in the second set as Djokovic crumbled.
The crowd were willing Williams on but sadly her inconsistency proved decisive in the next game, as two big serves were followed by two double-faults and a wide forehand that allowed the world number four to close out a dramatic victory.
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Stosur, who had nine double-faults and 56 unforced errors, has never gone beyond the fourth round at her home Grand Slam, lost in the first round last year and also lost in her first matches at warm-up tournaments this year in Brisbane and Sydney.
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