Daniel Altmaier pulled off a major upset on Tuesday, beating top seeded Alexander Zverev in a battle of German players to qualify for the second round of the Mexican Open.
The 27-year-old Altmaier and world's 57th ranked player defeated his countryman for the first time in three tries in 2:26 hours and will play Miomir Kecmanovic next.
Zverev, sixth in the ATP rankings, lost a seven-match winning streak against German players. His previous loss against one was in 2019, when he fell to Dustin Brow, in Stuttgart.
Earlier, second-seeded Holger Rune eased past American Michael Mmoh 6-2, 6-3 and advanced to the second round.
Rune, ranked seventh in the ATP, needed 92 minutes to dispatch Mmoh, who made his way into the main draw through the qualification tournament.
The 20-year-old Rune, from Denmark, reached the semifinals in Acapulco last year and now will meet Aleksandar Kovacevic, who took care of Jordan Thompson 6-3, 5-7, 6-4.
Earlier, fifth-seeded Stefanos Tsitsipas defeated Roman Safiullin 6-3, 6-4 and moved on to the second round where he will face Flavio Cobolli.
The 25-year-old Greek reached the semifinals last week in Los Cabos, Mexico, and is trying to win his first career 500 tournament.
Also Tuesday, Frances Tiafoe, seeded eighth, rallied to beat Max Purcell 4-6, 6-3, 6-0. He moved on to face Dominik Koepfer, who took care of Terence Atmane 6-4, 4—6, 6-0.
Tiafoe is the highest-ranked American player in contention in the tournament after fourth-seeded Taylor Fritz and seventh-seeded Tommy Paul were knocked out on Monday.
In another result, Casper Ruud outlasted Christopher Eubanks 6-4, 6-7 (5), 6-3 and moved on to the second round.
Ruud is going to play against Dusan Lajovic, who beat local Rodrigo Pacheco 6-2, 6-2.
