While he quickly calmed down and even managed to hold two match points on his opponent's serve, the damage was largely done as Harris eventually closed out the match 6-3, 6-7 (8-6), 7-6 (10-8).

It was an unfortunate ending for the Australian No.1, who was coming off the back of his third ATP title win in Wuhan.

Loading

The towering South African largely had his measure and wasn't broken throughout the match.

But de Minaur refused to go away, hanging tough in the second set tie-breaker when Harris had two serves to take the match before scrapping his way to a deciding set tie-breaker.

The loss means he'll miss out on a second-round match with countryman John Millman, who fought back from a set down against Adrian Mannarino.

Millman played the big points better to prevail 4-6, 6-3 ,6-4 in two hours and eight minutes.

Meanwhile Novak Djokovic reached the quarter-finals by beating Japanese wildcard entry Go Soeda 6-3, 7-5.

Djokovic, who is trying to win a title on his tournament debut for the 10th time, hit 10 aces and saved three of four break points.

Djokovic retired with a left-shoulder injury during his fourth-round match against Stan Wawrinka at the US Open but showed no lingering effects in Tokyo.

Djokovic, who is 42-8 this season with three titles in four finals, will next face either Yoshihito Nishioka and Lucas Pouille.

Also, Yasutaka Uchiyama beat Radu Albot 6-7 (7-2), 6-3, 6-4, and third-seeded David Goffin rallied to beat Pablo Carreno 1-6, 7-6 (10-8), 6-0.

AAP