Lautaro Martinez struck late to snatch last season's Champions League runners-up Inter Milan a 1-1 draw at Real Sociedad on Wednesday, denying the Spanish side a triumphant return to the competition after a decade away.

Brais Mendez's early goal split the sides in San Sebastian for most of the night but the hosts spurned several opportunities to build on their lead in an intense showing that left Simone Inzaghi's team bruised but not beaten.

The Basque side's performance was all the more notable given Inter's 5-1 derby thrashing of rivals AC Milan on Saturday which kept them top of Serie A.

Only Martinez's cool finish after 87 minutes rescued the Italian side from a setback at the start of their Champions League campaign.

"I'm not satisfied because we did not play as we planned," Martinez told Amazon Prime TV.

"We had a lot of troubles in the first half because we were facing a team who pressed us very high and we had difficulties to build from the back."

He continued: "We did not play well but it was important to avoid defeat at least because the group would have become more complicated."

Imanol Alguacil's Real Sociedad started out strongly, with Mendez drawing a save from Yann Sommer before Ander Barrenetxea hit the post in a frantic opening.

Real Sociedad may have waited 10 years to get back into the Champions League, but it only took them four minutes to go ahead.

The hosts pressed defender Alessandro Bastoni high to win the ball back on the edge of the box, with Mendez sweeping it home stylishly.

Inter managed to calm the game down but could not create dangerous openings of their own.

Robin Le Normand headed over before half-time when he might have doubled La Real's lead, and Sommer foiled Takefusa Kubo.

Inter's Swiss goalkeeper tipped a Mendez free-kick to safety and then produced a superb save to deny Mikel Oyarzabal early in the second half as the hosts came out as determined as they were at the start.

The visitors' night nearly got worse when referee Michael Oliver sent off Nicolo Barella for violent conduct as he tangled with Mendez but rescinded the card after watching a VAR replay.

Real Sociedad midfielder Mikel Merino's glancing header clipped the bar with 20 minutes to go as his side tried to put the game to bed.

Substitute Marcus Thuram found the net for Inter from Carlos Augusto's cross, but the Brazilian full-back was offside and it was ruled out.

Real Sociedad did not heed the warning and conceded in the final stages when substitute Davide Frattesi's raking pass found Martinez, who smashed home.

"We faced a very strong side tonight, we knew it, and we did well to remain in the game despite playing badly thanks to our goalkeeper," admitted Inzaghi.