Ayew broke the deadlock with a glancing header from Mathieu Valbuena's corner in the third minute of time added on leaving the Inter players shaking their heads in disbelief as the French side stretched their unbeaten run in all competitions to 16 games.
Inter coach Claudio Ranieri left Argentine striker Diego Milito on the bench, even though he is the only player to have scored for the team in their last seven games, netting all the goals in the 4-4 home draw against Palermo in Serie A three weeks ago.
"Milito had influenza for a week. I did not want to risk him," Ranieri told reporters.
"Marseille were pushing on the left flank and Joel Obi did a good job to give us more balance."
"The San Siro will be full and it will be an advantage for us," he said. "We can score two goals no problem.
"We played well, we had good counter-attacks, we had more opportunities and they scored on their first chance.
"We did not deserve to lose," Ranieri added.
'SMALL ADVANTAGE'
Marseille coach Didier Deschamps was cautious about his team's chances of making the quarter-finals.
"It's a small advantage. It's going to be complicated," he told reporters. "We will not be in a position to only sit back and wait."
Inter's best chance of the match came in the 11th minute when Steve Mandanda tipped a Diego Forlan half-volley over the bar.
"It's good for him, but it's even better for
