The French world number 12 was made to work against the German on Tuesday before eventually coming through in three sets.
"I was tired so it was a bit complicated for me," Tsonga is quoted as saying by the ATP's official website. "Fortunately I had an early break in the first set and I was able to hold my serve, but I knew it was going to be tough.
"Then I had this letdown in the second set. I was able to hang in there in the third set, and he ended up letting it go. It's maybe because I practised too much or I didn't manage the amount of effort I made."