Alexander Zverev has been stunned in an all-German third-round clash with Jan-Lennard Struff at the ATP Masters 1000 event at Indian Wells.

World No.3 Zverev was out of sorts in the 6-3 6-1 defeat.

The 21-year-old handed the first break to Struff with a backhand error to fall 4-2 behind and he never recovered as his frustrations grew.

Zverev had four chances to break back and rescue the set but a determined Struff stood firm to serve out the opener.

It was no better in the second for Zverev as he slammed his racket after a net error to concede break points.

The 28-year-old Struff, who had previously never beaten Zverev, unleashed a punishing forehand to earn a double-break 4-1 advantage, before Zverev conceded the match miserably with a double-fault after 70 minutes.

Struff moved on to face Canadian 13th seed Milos Raonic, who rallied back from a set down to overcome American qualifier Marcos Giron 4-6 6-4 6-4 for his place in the last 16.

Meanwhile, Croatian veteran Ivo Karlovic served 16 aces as he progressed to the last 16 with a 6-3 7-6 (7-3) victory over Indian qualifier Prajnesh Gunneswaran.