World number one Serena Williams credited an on-court coffee for her recovery after she stormed back from a first-set bagel to beat Italy's Flavia Pennetta in her opening match of the year at the Hopman Cup on Monday.

The 33-year-old American looked all at sea during the first 19 minutes of the early morning match in a sweltering Perth - but the caffeine hit did just the trick as her serve clicked into gear for a 0-6 6-3 6-0 win.

"I'm so jet lagged and Flavia played really, really well and yeah, everyone needs coffee every now and then. I needed coffee to get my feet moving," Williams said in an on court interview to roars of appreciation from the crowd.

"I wanted coffee with my bagel and whatever coffee it was, it was really good."

Williams had asked a ball girl to get her the drink but was initially unsure if it was allowed.

"I just asked them to get me a shot of espresso. I asked them if it was legal, because I've never done it before." she told reporters later.

The 18-times grand slam singles champion is aiming for a sixth Australian Open in Melbourne later this month and is using the round robin mixed team event as preparation for the year's first grand slam.

She watched on from the stands and pumped her fists in delight as compatriot John Isner beat Fabio Fognini 5-7 7-5 7-6 (4) to give the U.S. the victory, with the duo making it a clean sweep by staving off match points to win the mixed doubles.

The big-serving American duo snapped up the first set 6-2 but the Italians stormed back taking the next by the same score and had two match points in the deciding tiebreaker but Williams and Isner recovered to pinch it 11-9.

The Americans will take on the Canadian duo of Eugenie Bouchard and Vasek Pospisil in the evening session on Tuesday, while Czech duo Lucie Safarova and Adam Pavlasek will aim for a second Group A win against the Italians earlier in the day.

In Monday's evening session, the British pair of Heather Watson and Andy Murray will face French duo Alize Cornet and Benoit Paire in Group B aiming to match Poland's 3-0 success over Australia.

Source: Reuters