Playing in Bucharest due to restrictions affecting travel between England and Spain, Chelsea dominated possession as has been the norm under Thomas Tuchel, yet the Blues needed a moment of supreme individual skill from their experienced forward to get the win.
Chelsea attempted 403 passes in the first half, with Atleti limited to just one goal attempt before the break – their lowest total after 45 minutes of a Champions League match since 2016-17 – and the breakthrough came in the 68th minute when Mario Hermoso's sliced clearance fell to Giroud.
Worth the wait. Olivier Giroud with a spectacular finish. 🚲 pic.twitter.com/VT5ojrD2NS— Champions League on CBS Sports (@UCLonCBSSports) February 23, 2021
He sent a wonderful bicycle kick beyond Jan Oblak, and although the offside flag initially denied Chelsea, the goal was awarded after a lengthy VAR check as the visitors took a lead into next month's second leg.
Chelsea had been let off the hook after Edouard Mendy's error inside the opening two minutes, and Tuchel's team escaped unscathed from another mistake when Thomas Lemar sliced wide.
Mason Mount has been a standout performer under Tuchel's tutelage and the England international crafted Chelsea's first opening a minute later – Timo Werner failing to divert a right-wing cross goalwards.
Mount should have done better with a volleyed cross-shot in the 40th minute, but Chelsea would have been punished had Luis Suarez managed to connect with Angel Correa's cut-back.
Joao Felix blazed an acrobatic attempt over the bar shortly after the restart, but whereas he failed, Giroud applied the finishing touch.
Mount forced a wild clearance from Hermoso which dropped over Giroud's shoulder, but the 34-year-old adjusted himself brilliantly to meet it and send a sublime finish into the left-hand corner.
Chelsea were made to wait as VAR checked whether it was Mount or Hermoso who had made contact, with the latter eventually adjudged to have deliberately played the ball, ensuring Giroud was onside and able to secure a vital away goal.
What does it mean? Chelsea on the cusp
Chelsea are attempting to progress past the round of 16 for the first time since the 2013-14 campaign, when the Blues lost to Atleti in the semi-finals. However, they will have to play the second leg without Mount and Jorginho, who have been prominent figures in Tuchel's midfield so far but picked up suspension-enforcing yellow cards on Tuesday.
Nevertheless, Tuchel has plenty of strength in depth at his disposal, and Atleti’s usually stringent defence has kept just one clean sheet in all competitions since the turn of the year.
Like a fine wine…
Like many luxuries in life, Giroud seems to be getting better with age. Aged 34 years and 146 days, he is the oldest player to score in the knockout stages of the Champions League for Chelsea, and the oldest to do so for an English side since Ryan Giggs (37) in April 2011.
The former Arsenal striker has now scored six Champions League goals this season, his best return in a single campaign in the competition.
Tuchel did not hold back in his criticism of Callum Hudson-Odoi following Chelsea’s Premier League weekend draw with Southampton, demanding more effort from the youngster, who was subbed on and then taken off at St Mary's.
Three days on, however, Hudson-Odoi turned in a solid display in his new-found wing-back role. He attempted a joint team-high two tackles, had one shot and finished with an 85.4 per cent passing accuracy before being replaced by Reece James late on.
In a big clash in the upper echelons of the Premier League, Chelsea welcome Manchester United to Stamford Bridge on Sunday. Atleti, meanwhile, face Villarreal in LaLiga.