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Frank Lampard's side won the reverse fixture 3-0 earlier this month but it was made to work a lot harder for Wednesday's (AEDT) victory - its sixth in a row in all competitions.
Serhou Guirassy looked to have rescued a point for Rennes when he cancelled out Callum Hudson-Odoi's first-half opener in the 85th minute at Roazhon Park.
But second-half substitute Giroud won it in added time to ensure Chelsea will progress from Group E along with Sevilla.
