Gareth Bale starred as holders Real Madrid thrashed Swiss side Basel to begin the defence of their Champions League title in impressive style.
After a Marek Suchy own goal had put Real ahead, Wales winger Bale smartly lobbed the keeper before finishing.
Bale set up Cristiano Ronaldo to tap in, James Rodriguez hit a fourth and Karim Benzema swept in a late fifth. Derlis Gonzalez scored for Basel.
All but one of Real's goals came in a first-half in which they were rampant, before Benzema struck late on after exchanging passes with Ronaldo for Real's 1,000th goal in European football.
The win will come as a welcome boost for manager Carlo Ancelotti, with the Spanish side having made a disappointing start to the season in La Liga.
After beginning the campaign with a 2-0 win against Cordoba, they lost their last two games in La Liga - including a 2-1 defeat at home against city rivals Atletico last time out.
The result puts Real top of Group B, level on points with Liverpool after they edged Bulgarian side Ludogorets Razgrad 2-1 at Anfield.
Atletico lost to Real in last season's Champions League final and they made a losing start to the current tournament, with on-loan Fulham striker Kostas Mitroglou getting the decisive third for Olympiakos in Greece.
Kostas Mitroglou, on loan at Olympiakos from Fulham, has scored nine goals in 13 Champions League games
The hosts took the lead thanks to Arthur Masuaku's superb strike from just outside the area, before Ibrahim Afellay drove in a second midway through the first half.
Mario Mandzukic headed in for Atletico to halve the deficit, but Mitroglou restored Olympiakos's two-goal advantage. Antoine Griezmann scored late on for Atletico.
Olympiakos are level with Juventus on three points in Group A, with Carlos Tevez scoring his first Champions League goals since 2009 to give the Italian champions a 2-0 victory at home to Swedish side Malmo.
Former Manchester City striker Tevez slotted in the opener after running on to Kwadwo Asamoah's clever backheel, before the Argentina international curled home a 90th-minute free-kick.
Juventus striker Carlos Tevez ended his Champions League goal-scoring drought, which had stretched back to 2009
In Group C, Andre Villas-Boas's Zenit won 2-0 at 10-man Benfica. Hulk slotted in the opener for the Russian side before Benfica had goalkeeper Artur sent off. Alex Witsel then headed in the game's second goal.
Monaco also won in Group C, with Joao Moutinho's deflected second-half strike enough to give the French side a 1-0 win at home to Germany's Bayer Leverkusen.
Finally, Borussia Dortmund beat Arsenal 2-0 to go top of Group D, with Belgian side Anderlecht and Turkey's Galatasaray sharing a 1-1 draw.