Williams put Reading 1-0 up at Meadow Park in the 56th minute of their Continental Cup group game on Sunday, but Bethany Mead cancelled out her goal with an equaliser six minutes later.
But that only inspired the former Gunner – who only left north London in August – to immediately try for her second of the game.
As soon as the game restarted, Williams shot straight from the centre circle after spotting Arsenal goalkeeper Anne Moorhouse off her line. The 33-year-old's strike was perfectly executed, and the ball soared into the back of the net.
Reading held on for a 2-1 victory, meaning the Berkshire club join Arsenal on six points at the top of the Group One South standings.
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