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.

“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).

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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.