That proved to be of help for regrouping Fognini later on with two games in a row. He went on to trail 4-5 while leading 40-15, but let slip his advantage in the tenth game to lose 6-4 in the first set.

Federer continued his momentum for another break midway through the second set, before sealing his quarterfinal spot 6-3 following Fognini's double faults.

"I served better than him at important moments. This was actually the key of the match," said Federer.

"What is crucial in a match is who serves better. It was me today. He didn't start well as well, which is always a bad thing for him," he added.

Federer will take on Kei Nishikori, who marched past No. 7 seed Kevin Anderson 6-4, 6-4. The Japanese kept his London hope alive with the win, but he needs to reach the final for a chance.

John Isner, however, is set to miss the Finals after losing to Karen Khachanov in a tussle featuring two tie-breaks in almost two and a half hours.

Despite saving three match points before triumphing the tie-break 11-9 in the second set, the American serving machine failed to repeat that in the decider as the Russian sealed the win at 10-8.

Khachanov booked a quarterfinal encounter with No. 4 seed Alexander Zverev, who got past Diego Schwartzman 6-4, 6-2.

Defending champion Jack Sock shrugged off lucky loser Malek Jaziri from Tunisia 6-0, 6-4, establishing a duel against the 2018 Roland Garros runner-up Dominic Thiem.

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