San Francisco Frontiers - Djokovic becomes oldest Masters semi-finalist in Miami, Mensik, Fritz through

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.

Czech teenager Jakub Mensik marched into his first Masters semi when he defeated France's Arthur Fils 7-6 (7/5), 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 towering Czech's big serve was too much for Fils, who had come through three-set battles with Frances Tiafoe and Zverev and showed signs of tiredness.

"It feels incredible. I think the biggest result so far in my career, so I'm glad I just kept going since the first round," said Mensik.

"That's the key, to keep the focus during the two weeks, because it's always tough. The job is not done."

Mensik broke out to an early 4-1 lead but then let Fils back into the contest with the 20-year-old Frenchman forcing a tie-break.

But having survived that comeback from Fils, Mensik was rock solid in the second set as he powered his way to victory in 75 minutes.

Mensik, ranked 54th in the world, will next face American third seed Taylor Fritz, who came through an enthralling battle with Italian Matteo Berrettini with a 7-5, 6-7 (7/9), 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.

C.Lewis--SFF