Australian qualifier Alexei Popyrin has been knocked out of the Swiss Indoors with opponent Alexander Zverev progressing to the quarter-finals.

The second-seeded German beat Popyrin 6-4 6-4 in their second round match at the St. Jakobshalle in Basel on Thursday to secure his 51st win of the season.

German Zverev, chasing a fourth ATP title this season, lost only two first-serve points on the way to victory.

Meanwhile, Roger Federer recovered from a slow start to beat Jan-Lennard Struff 6-3 7-5 and reach the quarter-finals of his hometown tournament.

The top seed was broken in the second game of the match by the big-serving German but, after going 3-1 behind, the 37-year-old hit back to win five successive games in 13 minutes and take the first set.

The second set went with serve until the 11th game when Struff made two unforced errors, including a double fault on break point.

Federer comfortably held serve to complete his third victory over Struff this year and claim his 17th successive win at the tournament, a run stretching back to 2014.

After claiming the title in 2014 and 2015, Federer missed the 2016 tournament through injury but won it again last year.

Fourth seed Stefanos Tsitsipas, who last week became the first Greek to win an ATP title when he triumphed in Stockholm, swept aside Germany's Peter Gojowczyk 6-3 6-1 to continue his impressive season.