Juan Martin del Potro wary of Milos Raonic threat ahead of Miami quarter-final

Argentine beats Krajinovic on Tuesday as he bids to win back-to-back Masters titles

Juan Martin del Potro is in the quarter-finals of the Miami Open. Erik S Lesser / EPA

Juan Martin del Potro sent his devoted fans home happy on Tuesday by powering into the ATP Miami Open quarter-finals with a 6-4, 6-2 triumph over Serbian Filip Krajinovic.

Roared on by a partisan crowd, the 29-year-old Argentinian recovered from 1-4 down in the first set to reel off nine games in a row on his way to a last-eight match with Canada's Milos Raonic on Wednesday.

Del Potro's return to form and fitness saw him win the Indian Wells title and continue to impress at the last tournament in Key Biscayne before a move north to the NFL Miami Dolphins' home stadium next year.

"Playing here they make it special for the South American players and I am trying to enjoy the last time playing in Key Biscayne," Del Potro said. "I know many Argentine fans live here and are so close. I will miss them."

Windy conditions hampered players all day and the former US Open champion took time to find his groove in his first meeting with 27th-ranked Krajinovic.

"The weather was so bad to play tennis. We couldn't feel the ball 100 per cent. We couldn't hit from the baseline," Del Potro admitted. "I'm confident I will feel better tomorrow. The better weather will make me feel better on court."

Raonic, who easily saw off Frenchman Jeremy Chardy 6-3 6-4, lost to del Potro in the Indian Wells semi-finals and will be eagerly seeking revenge.

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"It will be a different match to the one we played in Indian Wells, different conditions," Del Potro said.

"Milos is improving and playing very solid. He has the potential to win the tournament. It will be a good match to test my fitness and see what level I'm at.

"I don't feel pressure. I am just going down my own road and seeing how far I can get. I am not thinking about ranking or anything. Just the last time here in Key Biscayne."

The big surprise on Tuesday was the exit of second seed Marin Cilic, who was beaten 7-6, 6-3 by American John Isner.

The Croatian's run was halted by big-serving Isner in just 87 minutes to set up a last-eight meeting with South Korean 19th seed Chung Hyeon.

It was a fine performance by Isner, the 14th seed, who rattled down 12 aces and was boosted by winning 85 per cent of points on his first serve.

All three of Isner's wins over the 2014 US Open champion have now come in ATP Masters events and once a first set tie-break was negotiated in windswept conditions, the 32 year-old confidently held his nerve to send Cilic home.

Isner has never won an ATP Masters event but this display has fueled hopes this could be his time.

"I think in these events, the format is very good for me: two-out-of-three sets," Isner said.

"In a situation like that, if I'm feeling fit and healthy, which I am, and serving like I did today, I can beat a lot of people playing like that."

Meanwhile, Alexander Zverev, the fourth seed, powered past an out-of-sorts Nick Kyrgios 6-4 6-4 to set up a meeting with Croatian Borna Coric in the last eight.

Australian firebrand Kyrgios has been struggling with an elbow injury and Zverev's power and precision was simply too hot to handle as the German reached the quarter-finals for the second year in succession.

Coric, who reached his first ATP Masters semi-final in Indian Wells earlier this month, has never got this far in Miami but showed his qualities once again with a battling 7-6 , 4-6, 6-4 triumph.

South Africa's Kevin Anderson saw off American youngster Frances Tiafoe 7-6, 6-4 and will play Spain's Pablo Carreno-Busta, who dispatched fellow countryman Fernando Verdasco 6-0 6-3.