Novak Djokovic turned on the style to take his place in the last four of the Miami Open with an impressive 6-3, 7-6 (7/4) win over American Sebastian Korda, becoming the oldest man to reach a Masters 1000 semi-final.

The 37-year-old Serb made a strong start against Korda, breaking early to get a first set advantage that he never let slip but Korda showed great resilience in the second set, taking the contest to a tie-break.

But the fourth seed, looking for his seventh Miami Open title but first since 2016, again came up strong, sealing the win with a powerful ace.

Djokovic, who will face Bulgarian Grigor Dimitrov on Friday, had no doubt what had been the key to his success after winning 84% of his first-serve points.

"One word, serve. I was serving very well, probably the best serving performance, not just here, but in a long time," he told reporters.

"Eleven aces, when I needed to find the first serve. It makes life easier on the court when you are feeling your serve. I needed it in the second set when I think Korda was feeling his ground strokes much better."

A victory at Miami would be a landmark 100th singles title for the 24-time Grand Slam champion and he said he's glad to be refinding a top level.

"I am obviously playing the best tennis I have played in quite some time. It's great when I experience tournaments like this and performances like this. It motivates me and encourages me to keep going for more," he said.