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Belinda Bencic and Roger Federer of Switzerland shake hands at the net after winning their mixed doubles match against Cameron Norrie and Katie Boulter of Great Britain during day two of the 2019 Hopman Cup.

Federer and Bencic beat Britain's Cameron Norrie and Katie Boulter in their opening round-robin tie, while the US lost to Greece on Monday despite Williams winning her singles tie.

Federer is defending his title at the Australian Open, the first Grand Slam event, which gets underway on Jan.14.

Two breaks of the serve in each set were enough for the 20-time Grand Slam victor to win 6-1, 6-1 in just 57 minutes, just over two weeks out from the year's first Grand Slam.

"As a player it's something that you would dream of playing Roger Federer", Williams told reporters.

"A bit of polish is needed but it was nice to come out and play a pretty clean match", Barty said.

"I am happy in all aspects of my game that things were working - foot work, offensive play, serve, return - it was all there".

BC-TEN-Hopman Cup, 3rd Ld-Writethru, 388Roger Federer enjoys impressive start to new seasonAP Photo XTC113, XTC139, XTC133, XTC138, XTC142, XTC105, XTC112, XTC102, XTC109, XTC104, XTC102, TKSK813Eds: Updates with Switzerland beating Britain 3-0, updates summary.

Serena Williams said she wants a selfie with Roger Federer for her Instagram page. AFP  GIUSEPPE CACACE
Serena Williams said she wants a selfie with Roger Federer for her Instagram page. AFP GIUSEPPE CACACE

Earlier in the day, Germany's star duo Angelique Kerber and Alexander Zverev also made encouraging starts to the new season.

World No. 2 Kerber beat Garbine Muguruza 6-2, 3-6, 6-3 to snap a five-match losing streak against the two-time major victor.

In her first tournament outing since the controversial loss to Naomi Osaka in the US Open final in September, Serena Williams struggled early and her groundstrokes were erratic in the first set.

Boulter was much improved in set two, but Bencic held serve to make it 6-6 and then raced through the tie-break, which she won 7-0.

Serena saved another break point in game five and that proved to be a turning point, breaking back in the very last moment in game 10 when Sakkari served for the set.

There was little between the pair in the third set until Zverev surged in the tiebreak to take the match 6-4, 4-6, 7-6 (7/0).

After the match, Zverev paid tribute to former world number three Ferrer, who will retire from the sport next year.

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At 1:20 p.m., the child was "held for continued observation" after doctors found that he had a fever of 103 degrees Fahrenheit. Meanwhile, Homeland Security Secretary Kirstjen Nielsen warns the country's immigration system is at a "breaking point".