Beijing - Rafael Nadal said he was braced for the "huge" serve of John Isner after Spain's world No 1 bulldozed his way into the China Open quarter-finals on Thursday.
The 16-time Grand Slam champion Nadal saw off the young Russian Karen Khachanov 6-3, 6-3 to set up a last-eight clash with Isner in Beijing.
The in-form American, the sixth seed, is renowned for his booming service game and Nadal expects to come under a barrage on Friday.
Isner was impressive in blasting his way past Argentina's Leonardo Mayer 6-0, 6-3 and Nadal said: "Tomorrow will be a tough one against Isner, he's playing unbelievably well.
"I saw him today, he played so aggressive, having a lot of success, hitting a lot of winners, returning very well.
"Everybody knows that he serves huge and it is so difficult to break him.
"But if you add that he is playing very well now from the baseline, he is one of the toughest opponents you can meet."
Nadal, 31, the top seed and coming off the back of a third US Open title, saved two match points against Frenchman Lucas Pouille in his opener in Beijing.
But he was always in control against Khachanov, 21, breaking him in the fourth game of the first set on the way to a routine victory.
In the women's draw, former world number one and fourth seed Karolina Pliskova became the latest high-profile casualty on Beijing's outdoor hard courts, going out in the third round to unseeded Sorana Cirstea.
Cirstea follows fellow Romanian Simona Halep into the quarter-finals after the second seed sent Maria Sharapova packing on Wednesday.
Cirstea, ranked 44 in the world, stunned the Czech Pliskova 6-1, 7-5 to reach the last eight.
She will play Latvia's Jelena Ostapenko, the Roland Garros champion, after China's top player Peng Shuai retired with a knee injury at 3-0 down in the first set.
The 31-year-old Peng said she had been having injections to ease the pain and her right knee was heavily strapped, but she could not play on.
"The doctor suggested I should take a period of rest and get it treated, but I can only rest after the season, that's the plan," said Peng, ranked 25 in the world.
World No 1 Garbine Muguruza and reigning champion Agnieszka Radwanska are both out, leaving world No 2 Halep as the favourite in the Chinese capital.
Also into the quarters are Caroline Garcia, who defeated French compatriot Alize Cornet 6-2, 6-1, and Barbora Strycova of the Czech Republic.
Garcia is in red-hot form after winning the Wuhan Open title on Saturday.