Roger Federer continued his mastery over Stan Wawrinka, but not before getting a fright en route to a 6-7, 7-6, 6-2 quarter-final victory at the Cincinnati Masters on Friday.
Federer upped his career record against his Swiss compatriot to 21-3.
A match that started with a low key atmosphere developed into a cracking affair when Federer took the second set tiebreak as lightning reverberated near the stadium.
Play was finally halted two points into the third set. Federer was in a different class once the match resumed after a short delay.
While Federer narrowly avoided an early exit, another high seed was sent packing, as No 4 Juan Martin del Potro fell to diminutive Belgian David Goffin 7-6, 7-6.
Goffin closed out a tight match with a backhand winner down the baseline.
His reward is a Saturday evening semi-final against Federer, while the other last-four clash will match former world No 1 Novak Djokovic of Serbia against Croatia's Marin Cilic.
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It was a busy day for 10th seeded Djokovic, who came from behind to oust defending champion Grigor Dimitrov of Bulgaria 2-6, 6-3, 6-4 in the conclusion of a third-round match carried over from Thursday.
Only a couple of hours later, Djokovic stopped Milos Raonic 7-5, 4-6, 6-3, improving his career record to a perfect 9-0 against the Canadian.
"I'm very pleased with my fighting spirit and staying mentally tough to overcome deficits in both matches," Djokovic said.
"Obviously the match against Dimitrov yesterday didn't start so well for me. He started tremendously well. We were not the only ones [dealing with interruptions], but it just makes it harder, because you're warming up, you're cooling down.
"We went home around 10.30, came back this morning again to play him, and then [had a] couple hours of rest.
"And to go back on the court again against Raonic, who was serving 140-plus miles [per hour] from the first game, that got me going, for sure, from the beginning."
Former champion Cilic made his way to the semi-finals with a 7-6, 3-6, 6-4 third-round win over Russia's Karen Khachanov before toppling Spaniard Pablo Carreno Busta 7-6, 6-4.
Earlier Friday, world No 2 Federer, bidding for his eighth Cincinnati title, outlasted Leonardo Mayer of Argentina 6-1, 7-6 in the third round.
Playing in his first tournament since Wimbledon, Federer lost just four first-serve points and did not face a break point in the match.
Mayer had a chance to force a third set, but Federer held on to win his third consecutive meeting with the Argentine as players double up in matches in the rain-delayed tournament.
Wawrinka beat Marton Fucsovics of Hungary 6-4, 6-3 in their third-round match.
Fourth seed Del Potro overcame Australian Nick Kyrgios 7-6, 6-7, 6-2.
Goffin did not face a break point in defeating sixth seed Kevin Anderson of South Africa 6-2, 6-4 in 76 minutes.