Miami Open: Nick Kyrgios out as John Isner upsets Marin Cilic

Alexander Zverev (right) beat Nick Kyrgios earlier this year in a Davis Cup tie

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.

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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.