Cologne had the worst possible start on home soil as Jonas Hector's return pass was intercepted by Kramaric, who kept his cool and nutmegged goalkeeper Timo Horn for the opener with only three minutes gone.
Top seed Djokovic labored for two hours to win 6-3, 4-6, 6-3 against German qualifier Dominik Koepfer.
"We had chances to score in the first half, but we missed all of our good chances," Tuchel said.
In the 28th minute, thanks to a short cross from Cedric Bakambu on the midfield, Renato Augusto dribbled through defender Ding Haifeng and contributed a calm tap-in to pull one back.
The Bayern striker indicated his club might use different line-ups and tactical approaches to get through the season.
18-year-old Lorenzo Musetti of Italy, who had upset three-time Grand Slam winner Stanislas Wawrinka of Switzerland and Japan's top player Kei Nishikori in the tournament's opening two rounds, lost to Dominik Koepfer of Germany 6-4, 6-0.
Middle blocker Wang Yuanyuan also returned from the national team to join Tianjin in their quest for the title.
But the partnership broke down as they failed to reach an agreement on controversies said to be mainly caused by the COVID-19 pandemic, PP Sports said.
Abel Hernandez and Gabriel Boschilia each scored twice for the Brazilian outfit while Duvan Vergara, Adrian Ramos and Santiago Moreno were on target for the visitors.
There were also seven goals in the game between the newly-promoted pair of Leeds United and Fulham, with Leeds winning 4-3 to claim their first points of the season.
The Curitiba-based side top the Group C standings with six points from three matches, ahead of Chile's Colo-Colo on goal difference. Jorge Wilstermann are third with three points.