Chelsea welcomed Atletico Madrid to Stamford Bridge in their Champions League last-16 second leg tie on Wednesday evening.
Having won 1-0 in Spain, Chelsea were the favourites to progress to the quarter-finals.
There were very few chances in the first 25 minutes as both teams cancelled each other out.
But there was controversy just before the half-an-hour mark when Yannick Carrasco went down inside the box.
The Belgian forward beat Cesar Azpiliceuta in the box and looked to be one-on-one with Edouard Mendy.
But Carrasco was seemingly denied a goal scoring opportunity when he crashed to the floor after contact from Azpilicueta.
However, the referee waved away his appeals. VAR was not even consulted.
View the moment below:
That is very controversial.
There's no doubt that there was contact there. Carrasco went down easily but you've seen them given.
Had that been given, Atletico would not only have had a golden opportunity to level the tie but Azpilicueta would almost certainly have received his marching orders.
Atletico's misery was amplified shortly after when the Blues took the lead.
Atletico threw men forward for a free-kick but they were punished.
Chelsea won the ball back and they launched a devastating counter-attack, which ended with Hakim Ziyech side-footing the ball past Jan Oblak.
There were no further goals in the first-half, meaning Atletico need to score at least two second half goals to remain in the competition.