San Francisco Frontiers - Czech teen Mensik, Djokovic into Miami semis

Fourth-seeded Novak Djokovic also took his place in the last four with a 6-3 7-6 (7/4) win over American Sebastian Korda.

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 54 in the world, will next face the winner of the quarter-final between Taylor Fritz and Matteo Berrettini later on Thursday.

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

Djokovic will face Bulgarian Grigor Dimitrov in Friday's semi-final.

R.Baker--SFF