The Brazil international's strike just after the hour mark brought looked to have set the Premier League side for a vital win.
But Lionel Messi's first goal against Blues in nine attempts handed the Blaugrana the advantage going into the second-leg at the Camp Nou.
Willian's been renowned for his blistering pace, but his well-taken effort proved how deadly the former Shakthar Donetsk man is from range, with his sixth goal from outside the box since joining the club in 2013.
Paris Saint-Germain forward Neymar and Cristiano Ronaldo come closest to the new Champions League record with five goals each from outside the area.
Despite struggling to break into manager Antonio Conte's starting XI in the Premier League, the 29-year-old has impressed in Europe with three goals and as many assists in seven appearances.