France's Gael Monfils, seeking his first ATP crown outside Europe, and Ivo Karlovic, nearing titles in consecutive weeks at age 37, advanced to the ATP and WTA Washington Open final in sweltering conditions Saturday.
The 29-year-old Frenchman, whose most recent of five career titles came in 2014 at Montpellier, ripped ill German teen Alexander Zverev 6-4, 6-0 in one hour.
"It was difficult conditions," Monfils said. "It was very warm. It was very difficult to keep the ball in. I changed up the pace a little bit and it was tougher for him to handle it."
Croatia's 35th-ranked Karlovic fired 14 aces to defeat US fifth seed Steve Johnson 6-4, 6-4 in a match where temperatures on the court reached 140 degrees Fahrenheit (60 Celsius).
"There was this moment when I just wanted to lay down on the court. But I kept going," Karlovic said. "It was difficult."
Monfils has split four career ATP matches against Karlovic, winning their most recent meeting at Madrid in 2011 but losing their only prior hardcourt matchup in 2009 at Cincinnati.
"There's only one Ivo," Monfils said. "Sometimes you only have one opportunity so you have to make the passing shot and then stick with the serve."
Karlovic, averaging 20 aces a match, has not surrendered a service break this week.
"This is my game. This is my strength," Karlovic said. "It is really working this week."
Zverev, ranked a career-best 27th but sick with food poisoning, warned Monfils could test Karlovic with quick returns.
"He's the quickest player on tour," Zverev said of Monfils. "He will get a lot of returns back."
Monfils, who seeks his first outdoor crown since Sopot in 2005, is 5-19 in ATP finals, including a 2011 loss at Washington to Radek Stepanek.
Karlovic, 7-8 in ATP finals, won his seventh career title last week on Newport grass to become the oldest tour singles champion since 1979.
"I'm confident from winning last week," Karlovic said. "In my old age, it's good when you can meet your goals still."
Karlovic broke in the third game of the match and took the first set in 38 minutes, ending it like the second with back-to-back aces.
In the third game of the second set, Karlovic rocketed a service return forehand winner past Johnson and then dropped a slice backhand winner on the line to break for a 2-1 lead.
"I tried to be as aggressive as I could and it paid off," Karlovic said. "In all the crucial moments I was able to make a good shot."