Washington - 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 (10/8), 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."
Saturday's other semi-final sends US eighth seed Jack Sock -- a 7-5, 6-4 winner over Canadian third seed Milos Raonic -- against South African 15th seed Kevin Anderson, who 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."
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.
"Nishikori's second-set drama intensified when Paul, in only his sixth career ATP main draw, hit a cross-court winner for an 8-7 tie-breaker edge. But Paul followed with a wide forehand, a backhand beyond the baseline and a miss-hit, handing Nishikori the set after 73 grueling minutes.
"For sure he got a little bit tight," said Nishikori. "But those points I played good points. I didn't make any unforced errors. Very focused. Experience worked well for me."
Nishikori, in his first event since a third-round Wimbledon exit, broke in the third game of the final set and held serve from there to eliminate Paul, who reached his first career ATP quarter-final last week in Atlanta.
"It was a tough match but it was a lot of fun," Paul said. "It was awesome. I didn't want to leave the court when I was done."
Nishikori hasn't won an ATP title in 18 months, making his 30th start this week since winning last year at Memphis, a run that includes six losses in finals.
Raonic's title drought reached 19 months and 30 events after falling to Sock. The 2014 Washington winner last lifted an ATP trophy in 2016 at Brisbane.
US eighth seed Sock won 12 of the last 14 points over Raonic in the first set and broke the Canadian in the penultimate game of the second on the way to an 85-minute victory.
"I was able to scrap out some balls and stay in some points to finish it off," Sock said. "It has been crazy humid and hot."
Sock has a 2-1 career edge on Anderson, winning their most recent meeting in last year's Auckland semi-final.