Novak Djokovic is through to the last four of the Miami Open after a 7-5 7-5 victory over Spain’s David Ferrer.
The Serbian will face John Isner in the second of Friday’s semi-finals, following the American’s 6-4 6-3 win over Japan’s Kei Nishikori.
Britain’s Andy Murray
in the first semi-final, which takes place at 18:00 BST.
Seven-times Miami champion
will play Spain’s
Carla Suarez Navarro
in Saturday’s women’s final.
Suarez Navarro upset German ninth seed Andrea Petkovic 6-3 6-3, while world number one Williams beat
Simona Halep 6-2 4-6 7-5.
Williams, who has held top spot in the women’s rankings for 110 weeks, said: “I hit more errors than I did in my last match, which I thought was impossible.
“I’m just not at my best level right now which is a little frustrating.”
Ferrer, who was celebrating his 33rd birthday, took the opening three games against Djokovic, but was unable to keep up the pace as the world number one fought his way back into the match.
“I wasn’t feeling the ball that great at the beginning, but I knew that I’m going to have my chances if I stayed committed, if I kept on working,” said Djokovic, 27.
“I knew that he’s a great competitor, that he grinds a lot, runs, and makes you play always an extra shot. That’s why he’s been top-10 player for so many years and he’s tough to beat.”
Isner became the first American man to reach the semi-finals since Mardy Fish in 2011, and has yet to be broken on serve this week.
“I played extremely well today,” said Isner.
“I needed to play well in order to beat a player like Kei and that’s what I did. From start to finish, I felt like I was aggressive. I was playing all the right shots and things just went my way.”