Despite the stunning upset victory of the Chinese qualifier Zhang Shuai over the current world No. 2 Simona Halep of Romania, the 2015 Shenzhen Open winner remains confident, saying she simply had an off-day at the Australian Open on Tuesday.

The second seed of the tournament lost to world No. 133 Zhang by straight sets 6-4, 6-3 in the first round of the touney that lasted one hour and 18 minutes.

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"She was better today. It's tough. I'm really disappointed. But it's only one day, only one match. Tomorrow, it's a new day," said Halep, who denied her injury played a part in the defeat.

Halep entered the Grand Slam tennis event nursing an Achilles injury and was reported to be suffering from a cold.

"I have to look forward and to stay motivated. You know, everyone can lose, everyone can win. I know that. I have just to take it like it is and look ahead."

Zhang has competed in a total of 14 grand slam appearances but she never won a single match and end up being defeated. However, with the sensational win over the tournament's second seed Simona Halep at Melbourne, the Chinese qualifier has finally able to break her considerable losing streak.

"Today is the best moment of my career ... beating a top-two player. I'm so happy, so excited," Zhang said.

Halep, 24, started tennis at a very young age of four. The Constantza native plays right handed (double handed backhand) and loves to play in the hard surface. She is currently under the watch of Coach Firicel Tomai.

Despite trying to be positive, it was a huge setback for Halep, who is enjoying her highest ever ranking after a stellar 2015 in which she won titles at Shenzhen, Dubai and Indian Wells.