To make things worse for Schalke, Leipzig continued on the front foot and made it 3-0 just before the break as Marcel Halstenberg's cross allowed Willi Orban to make it three.
"It was a very even match, perhaps more difficult than the result showed," said Djokovic, who is seeking for an 18th Grand Slam title.
The world No. 4, and two-time finalist at Roland Garros in 2018 and 2019, reached the fourth round in Paris for the five consecutive years.
Iran sit third, behind Iraq and Bahrain, in Group C of the Asian qualifiers for the FIFA World Cup Qatar 2022. Enditem
Argentina will host Ecuador at La Bombonera in Buenos Aires on October 8 before a clash with Bolivia in La Paz five days later.
The "Eisernen" made a perfect start on home soil as Christopher Lenz tested Mainz goalkeeper Robin Zentner in the early stages, before Max Kruse nodded home his debut goal after Sheraldo Becker's pinpoint cross to the far post in the 13th minute.
Barca issued a communique shortly after the final whistle confirming Alba had "problems in his right hamstring," adding that he would undergo tests on Monday to discover the full extent of the injury.
The match will be Iran's first game at the national level under the leadership of Dragan Skocic.
"They're the best team in the league, and proved it today," said Shandong coach Luka Sjekloca.
However, the six-time Ballon d'Or winner has not been able to win a major trophy at international level; a gold medal at the Beijing 2008 Olympics notwithstanding. The 2022 World Cup in Qatar might be his last chance to change that.
"We aimed at testing the team, as the two sides have a big gap. Our players executed our tactics in transition, but we made too many mistakes," rued Hebei coach Ma Tao.