The striker teamed up with fellow debutant Cesc Fabregas to cancel out Nik Kapun’s opener.
Another Blues signing, Kurt Zouma, scored the winner from close range after earlier having a goal ruled out for off-side.
After a drab start, Chelsea found their passing rhythm with Costa and Gary Cahill going close.
But against the run of play, the home side took the lead when Filip Valencic intercepted Isaiah Brown’s suicidal pass in defence and Kapun lashed the ball into the top corner.
Jose Mourinho’s side improved after the break and equalised when Fabregas threaded the ball through to Costa who fired the ball in on 55 minutes.
Eight minutes later, Fernando Torres’ shot was parried and Zouma stabbed in the rebound.
Torres then missed a sitter from two yards out and Branislav Ivanovic had a goal ruled out for a foul as Chelsea held on for victory.
21’ – COSTA CHANCE - Salah tries a shot in the box which is deflected out for a corner. The delivery falls to Costa whose header is comfortably gathered by the keeper.
25’ – DISALLOWED GOAL - From the free-kick, Matic flicks it onto Zouma who chests the ball down and fires it in but the ref has blown his whistle for off-side
26’ – GOOD STOP - Olmladic with a flicked header that is palmed away by Ratic, a fine save.
29’ – CAHILL SHOT - Cahill is inches away from scoring with a low shot from 12 yards out that whistles past the left-hand post.
31’ – BROWN EFFORT - Brown blazes over from 12 yards out with a shot that was already rising after controlling Costa's chipped pass.
40’ - GOAL! Brown with a suicidal pass and the home side capitalise with Kapun firing into the roof of the net from inside the box.
46’ – COSTA EFFORT - Costa drags his shot across goal from 25 yards.
49’ – RODIC SHOT - Rodic chases down a loose ball but his effort while stretching goes across goal.
55’ – GOAL! - Costa is through on goal with a diagonal run between two defenders and he fires the ball past the keeper to make it 1-1.
63’ – GOAL! - Zouma stabs in the ball from close range after Torres' shot is parried following a corner.
65’ – DISALLOWED GOAL - Ivanovic bundles the ball home after Zouma's header is brilliantly saved and the full-back dived in to push in the rebound.
76’ – TORRES MISS - How did Torres miss that? He chests the ball over from two yards after Ake's cross from the left.
78’ – GOOD EFFORT – A low drive from Burgic from distance which whistles wide.
80’ – DOUBLE CHANCE - Omladic’s low shot is saved by Cech, then Vukcevic’s volley is deflected inches wide.
88’ – SOLANKE MISS - Solanke knees the ball over after the keeper flaps at a cross by Ivanovic.
How did Cesc Fabregas and Diego Costa fare on their debuts? Despite some nice touches, Costa looked isolated in the first half and similar to the player who featured in Spain’s disastrous World Cup campaign. But he improved in the second half and took his goal well after ghosting between two players. Meanwhile Fabregas looked impressive as a sitting midfielder and executed some deft through balls. And he produced a trademark pass to set up Costa’s goal.
MAN OF THE MATCH
Kurt Zouma (Chelsea): His energy and power was there for all to see and he was a threat at set pieces. The Frenchman had a goal ruled out for off-side in the first half before getting on the scoresheet with a neat finish.
CHELSEA: Delac 5, Azpilicueta 5, Cahil 6, Zouma 9, Luis 7, Matic 6, Fabregas 7, Salah 6,Boga 5, Brown 4, Costa 6...Subs: Cech 6, Ivanovic 6, Ake 6, Terry 6, Chalobah 5, Van Ginkel 5, Romeu 5, Solanke 5, Bamford 5, Torres 4.
NK OLIMPIJA LJUBLJANA: Seliga 5, Jovic 5, Mitrovic 6, Delamea 5, Darijan Matic 5, Kapun 7, Omladic 6, Rodic 7, Sporar 5, Valencic 6...Subs: Ivacic 5, Zarifovic 5, Vidovic, Gajic 5, Durkovic 5, Zorc, Vukcevic 6, Bozic 5, Jurcevic 5, Golubar 5, Buric 5.
- Sports & Recreation
- Cesc Fabregas
- Diego Costa
- Kurt Zouma
- Fernando Torres