Mikel Arteta's men looked to be heading for an underwhelming goalless draw in Piraeus, but Lacazette struck nine minutes from time to get Arsenal a potentially vital away goal ahead of the return leg in London.

The Gunners had little difficulty taking control and should have netted early on when Lacazette was guilty of a dreadful miss, and the visitors failed to craft a better opportunity until late on.

Olympiacos generally appeared happy to hold on for a stalemate, but that approach backfired when Lacazette turned in Bukayo Saka's pass, putting Arsenal in complete control of the tie.

After surviving an early scare when Bernd Leno denied Mathieu Valbuena, Lacazette missed a glorious chance for the Gunners, somehow shooting wide from about eight yards having been teed up by Gabriel Martinelli.

Leno had to be alert again a few moments later, making a reaction save after Giorgos Masouras' cross was deflected goalwards by David Luiz.

A second opportunity came Lacazette's way just before the break, but Ousseynou Ba crucially got a touch to divert his fierce volley over the crossbar.

The contest became a cagier affair after the break, with Arsenal controlling possession but doing little to hurt an Olympiacos side seemingly content to absorb pressure.

But Arsenal finally produced some spark in attack through Saka in the 81st minute, the 18 year-old talent driving at the defence and feeding Lacazette, who tapped in from close range.

Sokratis Papastathopoulos almost doubled Arsenal's advantage when his late header came back off the crossbar.