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 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 Djokovic, 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 per cent 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.

Elsewhere, Czech teenager Jakub Mensik marched into his first Masters semi when he defeated France's Arthur Fils 7-6, 6-1.

Fils had enjoyed one of the biggest wins of his career on Wednesday when he defeated top-seed Alexander Zverev, but he struggled to repeat the feat against the impressive 19-year-old Mensik.

The Czech will next face American third seed Taylor Fritz, who came through an enthralling battle with Italian Matteo Berrettini with a 7-5, 6-7, 7-5.

Berrettini looked buried in the second set tie-break when he trailed 6-3 but his booming serve and some incredible returns gave him the opportunity of a deciding set. In total, Berrettini saved six match points in the second set.

It was nip and tuck with the pair on serve until Fritz, working hard on the baseline, broke to go 6-5 up and then held on for the win. (AFP)