Federer closed out victory two games later and will face David Goffin after the Belgian beat Juan Martin del Potro
World number two Roger Federer posted his second win of the day, amid a lightning fear interruption, to reach the semi-finals of the ATP-WTA Cincinnati Masters on Friday.
The seven-time champion beat fellow Swiss Stan Wawrinka 6-7 (2/7), 7-6 (8/6), 6-2.
The match was interrupted for 21 minutes with players being sent off court and crowds told to seek shelter as a storm cell passed near the site.
But after the pause, the pair came back out, with Federer finally securing the first break of the match on his seventh chance to take a 4-2 lead over his longtime rival.
Federer closed out victory two games later and will face David Goffin after the Belgian beat Juan Martin del Potro 7-6 (7/5), 7-6 (7/4).
One step closer to #️⃣8️⃣— Western & Southern Open (@CincyTennis) August 18, 2018
Seven-time champ @rogerfederer battles his way back to win all-Swiss battle with Stan Wawrinka 6-7(2) 7-6(6) 6-2 and reach his *EIGHTH* semifinal in Cincinnati. #CincyTennis pic.twitter.com/BIealPfftr
Federer began his day by beating Leo Mayer 6-1, 7-6 (8/6).
"It's quite unusual to stop for lightning," Federer said after beating Wawrinka for the 23rd time in 26 meetings.
"I didn't know if it meant the end of the night or that we would come back.
A day in the life of Roger Federer:— bet365_aus (@bet365_aus) August 18, 2018
Beat Leonardo Mayer in the morning.
Beat Stan Wawrinka in the evening.
The 37-year-old has just won TWO matches in the same day.
"I got my energy back for the third set, a momentum shift was good for me. It was also good that I was serving first.
"I played a cleaner third set. I was clear with my game plan. It was good to win two matches today."