Czech fourth seed Petra Kvitova said the standard of her serving was the contributing factor behind her surprise loss to Madison Keys.
Kvitova's struggles at Melbourne Park continued on Saturday as she was beaten 6-4 7-5 by American teenager Keys on Rod Laver Arena.
After reaching the semi-finals of the tournament in 2012, Kvitova has failed to make it past the second round.
And the 24-year-old's campaign this time around was cut short due in part to her serve.
Kvitova served at just 52 per cent and was broken five times throughout the match.
"For sure it was the serve. I mean, I didn't really expect that I can really put one serve in," the two-time Wimbledon champion told reporters.
"So it was really difficult for me, playing this kind of player when she has a big serve and I really need to hold my serve.
"I don't know what happened, but I couldn't put a serve in. So it was the main issue, for sure."
Kvitova did not fare much better on Keys' serve, winning just 24 of 60 points.
"She was going forward for it, playing real aggressive from the first point. I think that she played really, really fast, aggressively," Kvitova added.
"She didn't make a lot of mistakes at all. So it was really tough. I was trying to have a chance, but just didn't come probably. But still, I mean, the serve was really the big difference."