World number two Roger Federer won his second game of the day on Friday as he beat Stan Wawrinka to reach the semi-finals of the Cincinnati Masters.
The seven-time champion, who began his day with victory over Leo Mayer, beat fellow Swiss Wawrinka 6-7 (2-7) 7-6 (8-6) 6-2.
The match was interrupted for 21 minutes as the players and crowd left the court during a lightning storm, according to BBC.
"It's quite unusual to stop for lightning," Federer said.
"I didn't know if it meant the end of the night or that we would come back. But I got my energy back for the third set, a momentum shift was good for me."
After the interruption, Federer secured the first break of the match to take a 4-2 lead and closed out victory for a 23rd victory in 26 meetings with Wawrinka.
The 37-year-old will face David Goffin in the final four after the Belgian beat Argentine Juan Martin del Potro 7-6 (7-5) 7-6 (7-4).
Elsewhere, former world number one Novak Djokovic also won his second match of the day to set up a semi-final with Croat Marin Cilic.
The Serb followed up a third-round victory over holder Grigor Dimitrov earlier in the day with a 7-5 4-6 6-3 victory over Canadian Milos Raonic.
Djokovic, a five-time finalist in Cincinnati, is aiming to lift the trophy at the sole Masters 1,000 series event he has never won.
Cilic secured his semi-final place by defeating Spaniard Pablo Carreno Busta 7-6 (9-7) 6-4.
Meanwhile, women's top seed Simona Halep recovered from 4-1 down in the opening set to beat Lesia Tsurenko of Ukraine 6-4 6-1.
It was the second fightback of the day for Halep, who came from a break down in each set to overhaul Australian 16th seed Ashleigh Barty in the third round.
She will face rising 20-year-old Aryna Sabalenka of Belarus, who upset Madison Keys of the United States 6-3 6-4.
Two-time Wimbledon champion Petra Kvitova also reached the final four with victory over Belgian Elise Mertens 7-5 5-7 6-3.
The Czech will face Kiki Bertens after the Dutchwoman defeated Ukrainian fifth seed Elina Svitolina 6-4 6-3.
