Nishikori needed only four games to complete a 3-6 6-3 4-6 7-6(4) 6-4 defeat of Italian Simone Bolelli after their third round match had been halted by bad light on Saturday.
The 24-year-old 10th seed is enjoying the most consistent year of his career, reaching the last 16 of the Australian Open and the Madrid Masters final as well as claiming two ATP titles.
He said it was no coincidence that his form had improved after employing former French Open champion Chang.
"It's been great. Already it's six or seven months and yeah, I love how he coaches me. My tennis is also changing. You see my ranking is much higher than last year.
"So, obviously his help is something, for sure. I love how we are doing together right now."
Chang, born to Taiwanese parents, became a standard bearer for Asian tennis after winning the French Open in 1989.
Nishikori, now the new darling of Asian fans, is similar to Chang in physique and style and believes that he is now beginning to learn from the American's well-known mental toughness.
"It's not because we are Asian, but we are kind of the same in the we play," he said. "He wasn't the biggest guy on tour.
"He knows how I should play. But maybe also mentally he can understand a little more the Japanese way.
"He's teaching me to be mentally strong. He's very positive always. You know, I love his personality."
Nishikori will need plenty of resilience in his next match against big-serving Canadian giant Milos Raonic.
(Editing by Josh Reich)
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