"I believed in myself. If not, the fifth set is going to be different result," stated Zhang, who also needed five sets to navigate past J.J. Wolf in the first round.
"In the end of the match, finish the last point, I don't really celebrate, not super happy. But it is happy like inside, but it just didn't show up."
Earlier on Grandstand qualifier Dominic Stricker also pulled off a seismic upset.
The world No. 128, Roland-Garros boys' champion as recent as 2020, clattered 78 winners to blaze past No. 7 seed Stefanos Tsitsipas.
Five sets, four hours, three tie-breaks, it was exhilarating from start to finish for a 7-5, 6-7(2), 6-7(5), 7-6(6), 6-3 scoreboard.
"Great performance. I enjoyed it from the first moment on," stated the Swiss youngster, breaking new ground at Grand Slams.
"I was down break in the fourth, came back. Actually, a bit speechless what happened today.
"So for sure the confidence for myself is increasing, and it's just great to see that I have this level over five sets."