Gareth Bale's long-awaited Real Madrid home debut ended in disappointment as Atletico Madrid beat Los Blancos 1-0 to register their first league victory over Real for 14 years on Saturday.
Bale started on the bench and had to watch on as Koke played in Diego Costa to slot Atletico into an 11th minute lead.
The Welshman was then called upon by Carlo Ancelotti at half-time to turn things in Real's favour, but he wasn't able to impact the game greatly as Atletico maintained their 100 percent start to the campaign.
They have 21 points from seven games and are second on goal difference behind Barcelona who also have seven wins from seven after clinching a 2-0 win at Almeria.
However, Koke insists Atletico are not yet thinking about the title.
"We are very happy with the result today but also with the effort of the whole team," he said.
"We are having a great year and we need to continue like this. The manager gave us a few instructions and we managed to score first then hold out pretty well."
Madrid captain Sergio Ramos admitted his side need to reflect after a first defeat of the season, which leaves them five points off the leaders.
"When things don't go well, you need to reflect," he said.
"They have won the derby because we lacked the eficiency we have had in recent years. We lacked rhythm and movement between the lines."
After a cagey opening 10 minutes, the Madrid derby burst into life when Koke played Costa in behind the home defence and the Brazilian slotted home his eighth goal in seven league games.
Even the goal though couldn't shake Real out of their early malaise and it took until the 20th minute for Thibaut Courtois to be forced into a save as he palmed Karim Benzema's flicked header behind.
The French striker had a much clearer headed opportunity from another Angel di Maria pass six minutes later but this time failed to even find the target.
And it was Atletico who should have doubled their tally before half-time on more than one occasion.
Firstly, Tiago squandered a glorious chance as he headed over when completely unmarked from Koke's header before Diego Lopez made a fine save to deny Gabi and then Costa just couldn't bundle home the rebound as Real struggled to cope with another corner.
Ancelotti reacted to his side's flat first-half display at half-time as he introduced both Bale and Luka Modric with Asier Illarramendi and Di Maria making way.
However, Lopez again had to keep Real in the game with an excellent low block as Costa raced clear on goal 15 minutes into the second period.
Bale forced Courtois into his first stop after the break with a low drive which the Belgian handled confidently.
Cristiano Ronaldo then fired a free-kick wastefully over the bar and saw another effort from distance spilled by Courtois.
Koke was desperately unlucky not to seal the three points for Atletico 10 minutes from time when his curling effort from the edge of the area came back off the crossbar.
And Courtois had to come to Atletico's rescue moments later to beat away Alvaro Morata's acrobatic effort.
Morata also fired into the side netting from a narrow angle as the game entered stoppage time but Atletico held on to register their best ever league start.
Barcelona's win came at a cost as Lionel Messi was substituted during the first-half with a thigh injury.
He is now doubtful for Tuesday's Champions League clash against Celtic.
"Leo apparently has a muscular problem in his right leg which he suffered when scoring the goal. More than that, it is difficult to say until they do the tests," said Barca boss Gerardo Martino.
Messi had given Barca the lead with a stunning left-footed strike for his 11th goal of the season before being replaced.
And Adriano tapped home a second 10 minutes after the break from Cesc Fabregas' cross.
It is the first time in Barca's history that they have won their first seven league games.
However, Martino was quick to give the credit for the record-breaking start under his tutelage to the players.
"These boys have broken many individual and collective records over the past few years before I got here," he said.