WASHINGTON DC, USA - No.7 seed Ekaterina Makarova booked the first spot in the Citi Open semifinals after Simona Halep found herself having to retire early in the third set.

Halep had to mount a comeback the previous day after dropping the first set to an in inspired Mariana Duque-Mariño, who hit one of the best shots of the year against Halep in her 3-6, 6-4, 6-2 defeat.

But against Makarova on Friday, Halep could not find enough in the tank after dropping the second set to Makarova, retiring at 2-6, 6-3, 1-0 citing heat illness and sending the Russian into the semifinals.

The World No.2 took a wildcard into the Citi Open to start her North American hardcourt season early - a move that would boost her hopes of becoming the WTA World No.1. Halep could not overtake current No.1, Karolina Pliskova, this week, but a run to the final here would have put the top ranking in play next week at the Rogers Cup in Toronto.

The Romanian was bossing the first set, building up a 4-0 lead before running into the first signs of resistance from her opponent. A loose service game and double fault gave Makarova a break back at 4-2, but Halep stayed in control to break straight back and calmly serve out the opening set.

After trading breaks to start the second, Makarova refused to fade and hung in the rallies as Halep patiently weathered the assault. But as Halep continued to slow down and play increasingly more sluggishly in the balmy heat, Makarova surged ahead to win four of the next five points and send the match into a decider, where the Russian started with a break.

But during the changeover, Halep asked for the tournament doctor and, after receiving treatment, called time on the match, citing heat illness.

“Today was really hot, and I started so slow,” Makarova told reporters at her post-match press conference. “When I went on the court I felt like the sun just took away all my energy.

“Simona, she’s a great player and No.2 in the world, so it’s always tough to play against her. I was just trying to stay with her in every point with my game and fight. Just move as fast as I can.

“In the end it worked for me, but I’m so sad we weren’t able to finish the game. But this is Washington, here it’s always hot like this.”

Makarova moved on into her first semifinal of 2017, where she’ll take on No.5 seed Océane Dodin. The Frenchwoman survived a rollercoaster match against the big-serving Sabine Lisicki, coming back from a set down to win 5-7, 6-4, 7-6(4).

Rounding out the Citi Open semifinals will be an all-German clash between Fed Cup teammates Julia Goerges and Andrea Petkovic. Goerges, the No.4 seed, edged past Monica Niculescu in straight sets, 6-3, 6-4, while Petkovic overcame Canadian wildcard Bianca Andreescu 6-7(3), 6-1, 6-2 to reach her first WTA semifinal of the year.