In hindsight, perhaps Roberta Vinci's victory over Serena Williams at the 2015 U.S. Open women's semifinals wasn't as much of a head-scratcher as originally believed.

The reigning queen of tennis has lost her last three matches, including the loss to the Italian at Flushing Meadows. Williams' latest defeat came in her 2015 International Premier Tennis League debut as she fell 6-4 to world's No. 11 Karolina Pliskova in a team competition, according to WTATennis.com.

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To be fair, the "match" was only one set and nothing more than an exhibition -- in terms of her 2015 record. It does not count as an official loss.

Nor does her other loss since the U.S. Open semis, a 6-3, 6-4 defeat at the hands of BFF Caroline Wozniacki in Denmark last week.

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But for those counting, that's three straight sets dropped by the world's top-ranked player, who was on her way to becoming the first player in 27 years -- man or woman -- to capture the calendar Grand Slam.

Williams dropped out of sight following her U.S. Open loss, and her coach, Patrick Mouratoglou, said she was injured more than the public was led to believe and that she lacked the motivation to finish the 2015 season and appear at the WTA Finals.

Williams also recently revealed that her dog of 17 years passed away.

Whatever the case, Serena clearly is not in peak form. Her last three defeats have come to players ranked 41st (which Vinci was when she beat Williams), 17th (Wozniacki) and 11th (Pliskova). There's no sense of panic in the Williams camp, as she still is considered head and shoulders above the rest of her WTA competitors.

If Williams still is losing to start 2016, then panic can begin to set in. Still, Mourtoglou may have been correct in questioning the tennis star's level of desire after her bitter defeat in New York.

But at 34 years old, Williams soon could be in danger of facing what other elite athletes -- Tiger Woods, Kobe Bryant and Peyton Manning are facing -- a losing battle against time.

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