Williams was all at sea as she was bagelled in the opening set of her match with Flavia Pennetta, so she ordered an espresso, and that did the trick as she recovered to win 0-6 6-3 6-0.
John Isner then beat Fabio Fognini 5-7 7-5 7-6 (7-4) and the pair then teamed up to win the mixed doubles ??? with a Champions tie-break decider – 6-2 2-6 11-9,
Williams said: “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.
“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 hot drink fix but was initially unsure if it was allowed.
“I am a coffee drinker. I didn’t have mine this morning and I was just feeling it, so I just had to get some coffee into me,??? she said.
“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.”
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 victory over Australia.