Meanwhile Rafael Nadal beat Serbian Miomir Kecmanovic 6-2 7-5 to reach the quarter-finals in Mexico.
”He’s a very dangerous player, young, with a lot of power. I’m very pleased with the victory,” Nadal told reporters post-match.
I played a very good first set and that’s important news for me. That gives me confidence.
The Spaniard will now play South Korea's Kwon Soon-woo in the last eight.
Should Nadal win the title Novak Djokovic must reach at least the semi-finals of the Dubai Duty Free Tennis Championships, also taking place at the moment, for the Serb to retain the world number one spot.
British number one Kyle Edmund secure an emphatic straight-sets victory over Canadian rising star Felix Auger-Aliassime, beating the 19-year-old 6-4 6-4 to set up a meeting with American Taylor Fritz.
Meanwhile fellow Brit Heather Watson saw off Ukraine's Kateryna Bondarenko 7-6 (7-2) 6-2.
The world number 69 saved two set points at 5-4 down before taking the set in the tie-break. In the second she raced to a 3-0 lead and never looked back en route to the quarter-final where she will face American Christina McHale.
It was a bad day at the office for 23-year-old Katie Boulter, though, who slumped to a 7-6 (7-2) 7-5 defeat by sixth seed Zhu.