Celtic were held to a 1-1 draw away to Slovenia's Maribor in the first leg of the Champions League playoffs, while Porto took control of their tie against Lille after securing a 1-0 win in France.
Celtic were reinstated to Europe's premier club competition after Poland's Legia Warsaw were punished for fielding an ineligible player.
Two-time European champions Porto will take a 1-0 advantage into next week's second leg against Lille in Portugal.
