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Germany's Alexander Zverev beat Australia's Nick Kyrgios to reach the quarter-finals of the Miami Open.

Kyrgios, 22, had progressed to the semi-finals of the tournament for the last two years but lost 6-4 6-4 against the world number five.

The Australian, who traded social media insults with Spain's Fernando Verdasco this week, complained of a sore back during the match.

Meanwhile, American John Isner upset second seed Marin Cilic 7-6 (7-0) 6-3.

Cilic, Croatia's 2014 US Open champion, was the highest-ranked player left in the draw following Roger Federer's loss to world number 175 Thanasi Kokkinakis.

"I am so happy to stay in and I am playing so well this year," said 14th seed Isner, who faces Hyeon Chung next.

South Korea's Chung beat Portugal's Joao Sousa in straight sets.

Isner, 32, who reached the Miami Open semi-finals in 2015, did not face a break point during the match, which lasted one hour 27 minutes.

He hit 12 aces in a dominant performance and won 85% of points on his first serve.

Indian Wells champion Juan Martin del Potro beat 22nd seed Serbian Filip Krajinovic 6-4 6-2 to advance.

Argentine fifth seed Del Potro, who won his first Masters title in California, has now won 14 consecutive matches.

Elsewhere, 20th seed Milos Raonic beat France's Jeremy Chardy 6-3 6-4, while Raonic's Canadian compatriot Denis Shapovalov was knocked out by Croatia's Borna Coric, who won 7-6 (7-2) 4-6 6-4.

Verdasco lost 6-0 6-3 to fellow Spaniard Pablo Carreno Busta.