Schalke fought back from a two-goal deficit to earn a 2-2 draw with Arsenal on Tuesday and stay top of Group B in the Champions League.
Theo Walcott and Olivier Giroud scored inside the first 30 minutes to put Arsenal ahead.
But a goal from Klaas-Jan Huntelaar in first-half injury time and another from Jefferson Farfan after the break completed the comeback by the German club.
"At 2-0 you have not won the game and with the physical quality of the German team it isn't over, they have a lot of quality and they never give up, so overall I never felt that the game was over even at 2-0," Arsenal's manager Arsene Wenger said.
The draw left Schalke with eight points after four matches, one ahead of Arsenal. Olympiakos has six points after winning 3-1 against Montpellier, which has one.
"To get a 2-2 after 0-2 speaks for our team," Schalke midfielder Lewis Holtby said. "We showed our morale. But we could have lost at the end."
Schalke won 2-0 in London two weeks ago, but it was Arsenal which struck twice early Wednesday.
Walcott scored in the 18th minute after a mistake by Schalke midfielder Roman Neustaedter, who tried to head the ball back to his defenders but only put Giroud through. His effort was blocked by Benedikt Hoewedes, however Walcott picked up the loose ball to score into an empty net.
Giroud doubled Arsenal's lead in the 26th, connecting with a perfect cross from Lukas Podolski to score with a diving header from five meters (yards) out.
Huntelaar, who missed several chances throughout the match, scored his fourth goal in four games in the second minute of stoppage time in the first half after Walcott had slipped on the pitch and lost the ball.
Holtby put Huntelaar through, and the striker sent a low shot inside the far post, beating goalkeeper Vito Mannone.
Farfan leveled in the 67th as Schalke pressed forward. A cross from Ibrahim Affelay was intended for Holtby, but he miscued his header and the ball fell kindly to Farfan at the far post. His powerful shot was going into the net before being deflected by Thomas Vermaelen into the net.
Before that, Huntelaar had beaten the offside trap to go through alone but his shot hit Mannone, who was already on the ground.
In stoppage time, Schalke goalkeeper Lars Unnerstall stopped Walcott's point-blank shot.
"We played probably our best football this season. I have to compliment my team for coming back," Schalke coach Huub Stevens said.
Schalke lost right back Atsuto Uchida to an injury in the first half, and his replacement Marco Hoeger hobbled off in the second half.