Top-10 rivals Kei Nishikori and Alexander Zverev advanced to a semi-final showdown at the ATP and WTA Citi Open on Friday while second-ranked Simona Halep retired with heat fatigue.

Ninth-ranked Nishikori, the 2015 champion on the Washington hardcourts, saved three match points in a tension-packed second set before rallying to beat 225th-ranked American 20-year-old Tommy Paul 3-6, 7-6(8), 6-4 after two hours and 40 minutes.

“It was a great battle,” Nishikori said. “He almost had it. Happy to win.”

Zverev, a 20-year-old German seeking his fourth ATP title of the year and ranked a career-high eighth, took only 57 minutes to eliminate Russia’s Daniil Medvedev 6-2, 6-4.

“I felt so good from the start,” Zverev said. “It was a great match for me. It’s just all coming together now.”

It will be the first career meeting between Nishikori and Zverev.

“I’m sure it’s going to be a tough match,” Nishikori said. He has been playing really well.”

“Kei has been in the top-10 for a long time now,” Zverev said. “He’s going to be a tough test on the hardcourts.”

In Saturday’s other semi-final, the eighth-seeded American Jack Sock will play against South African 15th seed Kevin Anderson. Sock won won 7-5, 6-4 over Canadian third seed Milos Raonic in their quarter-final on Friday. Anderson, meanwhile, blasted 21 aces to defeat 200th-ranked Indian qualifier Yuki Bhambri 6-4, 4-6, 6-3, after ousting top seed Dominic Thiem on Thursday.

“I definitely have the belief in myself that I’m able to win this tournament,” Anderson said. “If I play my best tennis I have a chance to do very well.”

Crazy, hot and humid

A day after complaining that severe heat left her feeling “a bit dead,” French Open runner-up Halep quit her quarter-final match in even more scorching conditions, allowing Russian seventh seed Ekaterina Makarova to advance 2-6, 6-3, 1-0.

“It was just the heat. I felt a little bit sick and I couldn’t continue,” Halep said. “I had a headache and I felt sick.”

The top-seeded Halep broke Makarova three times to take the first set in 30 minutes, but the Russian battled back to win the second set and broke to open the third before Halep retired.

“The sun took so much energy,” Makarova said. “After the first set I started putting ice to my face because it was burning.”

Halep, who defends her title next week in Toronto, played the day’s first match three times this week, hitting the most humid and sweltering conditions. Organizers denied her request for a late match Friday.

Next for Makarova is a Saturday semi-final against 20-year-old French fifth seed Oceane Dodin. The other semi matches Germans Julia Goerges and Andrea Petkovic.