Del Potro, who arrived at the tournament having won the Indian Wells Masters, dispatched his Japanese opponent 6-2, 6-2 at the Crandon Park Tennis Centre in this Association of Tennis Professionals World Tour Masters 1000 event. 

The Argentinian is next due to take on Serbia's Filip Krajinović, who defeated Novak Djokovic's conqueror Benoit Paire of France 6-3, 6-3, in the next round.

Marin Cilic of Croatia, the highest seed left in the tournament following Roger Federer's shock exit yesterday, also progressed as he beat Canada's Vasek Pospisil 7-5, 7-6.

Cilic, the second seed, is now scheduled to face American John Isner for a place in the quarter-finals.

Isner joined the Croatian in round four with a routine 6-4, 6-3 win against Russian Mikhail Youzhny.

In the women's draw, defending champion Johanna Konta secured her fourth-round berth as she defeated Elise Mertens of Belgium 6-2, 6-1 in a match which only lasted 65 minutes.

The Briton's reward for the dominant victory in this Women's Tennis Association Premier Mandatory tournament is a meeting with American Venus Williams, who survived a scare when she beat Dutchwoman Kiki Bertens.

Williams dropped the first set and saved three match points in a dramatic contest before prevailing 5-7, 6-3, 7-5.

"I'm not sure how I did that," the 37-year-old said.

"I had a lot of chances, she had a lot of chances.

"It didn't look good sometimes but that's why you play to the last point. We were in a battle but I'm happy to be coming back tomorrow."

Elsewhere, Ukraine's Elina Svitolina also needed three sets to progress as she knocked out Australia's Daria Gavrilova 4-6, 6-0, 6-1.

Svitolina is next due to play another Australian, Ashleigh Barty, in the last 16.