Melbourne: Defending champion Serena Williams dispelled any doubts about her fitness and form after a four-month break from the game with a tense 6-4 7-5 win over feisty Italian Camila Giorgi to reach the second round of the Australian Open on Monday.

The world number one had barely swung a racquet in anger since her U.S. Open exit last September and entered a sweltering Rod Laver Arena to face the highest-ranked unseeded player in the draw amid speculation about the condition of her left knee.

Williams pulled out of her first match at the Hopman Cup in Perth earlier this month due to knee inflammation but the 34-year-old moved freely during a testing opener against Giorgi and said it gave her no problems.

"(The knee is) great. It was an hour and 43 minutes and I didn't feel it at all," Williams told reporters.

"I think I served well today. I think ... I got broken once, but other than that I was able to stay focused on that part."

Showing none of the lethargy that has sometimes dogged her in early grand slam matches, a calm and focused Williams roared to a 4-1 lead as the temperature soared over 32 degrees Celsius (90 Fahrenheit).

But 36th-ranked Giorgi, a slightly built player packing a meaty forehand, steadied herself to drag Williams into a baseline battle and the American's screams of frustration underlined the growing tension as the match progressed.

Though a match for Williams in terms of court speed and fire-power, Giorgi landed barely a third of her first serves and gave up the decisive break at 5-5 in the second set with her 12th double-fault.

Williams needed no further invitation and closed out the match with a customary barrage of booming first serves.

A shock semi-final upset at Flushing Meadows by unseeded Italian Roberta Vinci ended Williams' bids for both a 22nd major title and a rare calendar grand slam.

The loss plunged the American into a period of introspection but the win over Giorgi showed she has lost none of her competitive fire.

Williams, who next faces 90th-ranked Taiwanese Hsieh Su-Wei, said the break from the game helped her a lot.

"I have been going non-stop since the London Olympics, and seeing that this is another Olympic year, I kind of wanted to start the year out really fresh and really go at it again as hard as I can," she added.

"I just needed that time to just recover the best of my ability and get really fit ... and really train and get ready for the season."