The Argentine levelled for Barcelona 15 minutes from time at Stamford Bridge, after a wayward pass from Andreas Christensen was pounced on by Andres Iniesta.
Nevertheless, Willian saw enough in Chelsea's performance at Stamford Bridge to suggest they can upset the odds in the second leg.
He fine-tuned his radar after the interval, however, rifling home into Ter Stegen's bottom-left corner to give Chelsea the lead.
The second leg of Chelsea and Barca's Champions League round of 16 will take place on March 14.
Cesc Fàbregas praised the head coach's tactics - Willian struck the post twice before easing his team ahead - but urged his team-mates to be ruthless in the return. And Chelsea's organisation and willingness to contest every situation frustrated Barcelona, even the world-class pairing of Messi and Luis Suarez.
If Chelsea had chose to go with say, Alvaro Morata up front with Hazard and Willian on the flanks, the Blues would not have been able to get the hold of midfield.
Elsewhere, the night's other game produced a far more conclusive outcome with Thomas Mueller and Robert Lewandowski both scoring twice as Bayern Munich beat 10-man Besiktas 5-0. "He played a top game and I am very pleased for his performance".More news: Unemployment rises but wage growth accelerates
"But we know very well against opponents like these that if you make a mistake you pay".
Christensen's mistake was the only one made by Chelsea as they largely held the La Liga leaders at arm's length for the majority of the match. "We had the game under control when they scored their goal". "But we managed to get an error out of them and get the equaliser and it is a good result in the end", he added.
"Our three strikers [Hazard, Willian and Pedro] played a fantastic game".
Chelsea won 2-1 in Madrid in September and Willian said: "We can go there (to the Nou Camp) and do a great job again.
Now we have to think about the games coming and then we have to think about Barcelona".
Chelsea could have it all to do now in the second leg at the Nou Camp.