Callum Hudson-Odoi's 22nd-minute opener was canceled out by Serhou Guirassy with just five minutes of normal time left, and the match was supposedly headed for a draw.
However, Giroud, who had come off the bench in the 70th minute, rose the highest to connect with a rebound from Timo Werner's effort and headed home a 90th-minute winner to secure all three points for the Blues.
Chelsea Player Ratings
Edouard Mendy - 7.5/10
Mendy returned to his old stomping ground for the first time since leaving for Chelsea in the summer. even though there were no fans to welcome him, there was no love lost between him and his former colleagues.
They tested him constantly, but he stood tall, repealing everything thrown at him. However, Guirassy's corner was impossible to keep out.
Cesar Azpilicueta - 7/10
The Chelsea skipper had another fine evening and came up trumps defensively to keep Rennes at bay for most of the match. He made three successful tackles and clearances.
Kurt Zouma - 7/10
Zouma complimented Thiago Silva well in the defence. He showed excellent positional awareness and strong aerial prowess by winning four duels, the most in the match. The Frenchman also made four clearances for Chelsea.
Thiago Silva - 7.5/10
The veteran was flawless once again and brought his wealth of experience and leadership qualities to keep everything sorted at the back. He registered six clearances and interceptions, both of which were the highest in the match by any player. Clutch.
Ben Chilwell - 7/10
Chilwell continued his fine start to his Chelsea career with another solid performance. He got into good attacking positions in the first-half and despite playing most of the second in his own half, he was sound defensively.
Mateo Kovacic - 6/10
The Croatian was strong on the ball and made some good passes, but there was nothing spectacular from him today.
Jorginho - 6.5/10
His excellent distribution kept Chelsea ticking, while he also dropped back during the last phase of the match when Rennes piled on the pressure. He made one key pass and five successful tackles, the most in the match.
Mason Mount - 7.5/10
Mount created the opening goal by winning back possession and laying a stupendous cross for Hudson-Odoi to register his first European assist. His work-rate in the opening stanza was superb, but slightly receded after the break.
Callum Hudson-Odoi - 8/10
Back on the scoresheet after four weeks, Hudson-Odoi produced a wonderful finish beyond Gomis for the opening goal after sprinting almost 30 yards. Composed and nonchalant, just as you like.
Tammy Abraham - 6.5/10
Abraham's scoring run ended after a rather quiet performance from the young striker, but his positioning and hold-up play was excellent.
Timo Werner - 6/10
Werner contrived to miss an absolute sitter in the fourth minute, as he blazed his effort into row Z from just a few yards out with the goal gaping. He then missed another chance towards the end, but luckily for him, his effort came off Gomis and set-up Giroud to score the winner.
Olivier Giroud - 8/10
The Frenchman came off the bench and headed in the winner at the death to save Chelsea their blushes. He lept superbly to connect with the ball.
Hakim Ziyech - 7.5/10
Ziyech was subbed on to provide Chelsea with some attacking impetus and did exactly that, playing Werner in with a nice through ball which culminated in Giroud's winning goal.
Kai Havertz - 6/10
Coming on for Kovacic for the final 15 minutes of normal time, Havertz was quiet in the match.
Reece James - N/A
He was sent on in the second minute of stoppage time just to see out the rest of additional time.Published 25 Nov 2020, 03:52 IST