The 31-year-old Spaniard, who claimed his third US Open title last month, triumphed 6-3, 6-3 at the Olympic Green Tennis Center in Beijing.

He broke his opponent, ranked 42^nd in the world, in the fourth game of the first set and appeared in complete control thereafter.

Khachanov registered seven double faults and failed to convert any of his six break points.

Next up for 16-time Grand Slam champion Nadal is the United States’ John Isner, who defeated Argentina's Leonardo Mayer 6-0, 6-3.

Also through to the last eight is Russia’s Andrey Rublev after he overcame seventh seed Tomáš Berdych of Czech Republic 1-6, 6-4, 6-1.

In the women’s draw, Romania’s Sorana Cirstea claimed a shock 6-1, 7-5 win over fourth seed Karolína Plíšková of Czech Republic.

Cirstea’s reward is a meeting with Latvia’s Jelena Ostapenko, who progressed after her Chinese opponent Peng Shuai retired with a knee injury when 3-0 behind in the first set.

Barbora Strýcová will go up against fellow Czech Petra Kvitová in the quarter-finals after both came through their encounters today.

Strýcová beat Australia’s Daria Gavrilova 6-0, 6-4, while 12^th seed Kvitová defeated fifth seed Caroline Wozniacki of Denmark 6-1, 6-4.

Elsewhere, France’s Caroline Garcia reached the last eight with a 6-2, 6-1 victory over compatriot Alizé Cornet and now goes onto face Ukrainian third seed Elina Svitolina, a 6-2, 7-5 winner at the expense of Russia’s Elena Vesnina.