Britain's Johanna Konta was beaten 6-4 7-6 (7-1) by world number two Simona Halep to exit the Cincinnati Open in the quarter-finals.
Konta, who beat the Romanian in the last eight of Wimbledon, took an early lead in a first set that featured five breaks of serve before Halep triumphed.
The Romanian went 4-1 up in the second and, despite Konta saving five match points, took the match on a tie-break.
Nick Kyrgios beat Spaniard Rafael Nadal 6-2 7-5 in the last eight.
Nadal, 31, will return to world number one on Monday for the first time since July 2014, taking over from the injured Andy Murray, but he had little answer for his Australian opponent.
Kyrgios, 22, raced into a 4-0 lead with a double break and took the first set in 25 minutes, dropping only three points on his serve.
He served for the match at 5-4 up only for Nadal to break but it proved to only delayed his victory as he broke back and completed victory with his 10th ace.
Kyrgios will play Spain's David Ferrer, who beat Dominic Thiem 6-3 6-3 in the semi-finals, while the other last four match will see Grigor Dimitrov play John Isner.
Bulgarian Dimitrov eased past Japan's Yuichi Sugita 6-2 6-1, while Isner overcame fellow American Jared Donaldson 7-6 (7-4) 7-5.
Konta efforts in vain
The Briton had a break point in each of the Romanian's games in the opener but converted only two out of 10.
Konta then saved two match points at 5-3 down on her opponent's serve and three more from 0-40 down on her own serve when 6-5 behind.
But Halep easily claimed the tie-break to win a match that lasted one hour and 49 minutes and she will now face American Sloane Stephens in the semi-finals.
The other last four contest will see Wimbledon champion Garbine Muguruza face Karolina Pliskova.