TORONTO: Top seed Alexander Zverev switched into survival mode to claim a milestone 500th ATP match win on Thursday, fighting back to beat Matteo Arnaldi 6-7 (5/7), 6-3, 6-2 at the Toronto Masters.

Germany's Zverev broke twice in the deciding third set to reach the fourth round of the US Open tuneup, converting his second match point to back up his defeat of the Italian ealier this year in Acapulco.

"It's a great achievement," Zverev, winner of 24 ATP titles, said of reaching 500 career match wins. "Not a lot of player reach this milestone.

"But I still want 500 more – maybe even more," he joked. "You always want to win as many matches as possible. I'm all about this."

Unable to gain the upper hand in an 82-minute first set, Zverev said his game improved as the minutes dragged on against 2024 Canadian semi-finalist Arnaldi.

"I started playing better from the baseline and found my rhythm a bit. My backhand felt off but I was comfortable and happy with my level," added Zverev who next faces Francisco Cerundolo – a 6-3, 6-4 winner over fellow Argentine Tomas Echeverry.

Alex Michelsen reached a Masters 1000 fourth round for the first time with a 3-6, 7-6 (7/4), 6-4 upset victory over third-seeded Lorenzo Musetti.

The 20-year-old American survived nearly 50 unforced errors and converted just two of nine break chances, but it was enough to vanquish Italy's Musetti, a two-time Grand Slam semi-finalist ranked 10th in the world.

It feels really good," the 26th-ranked Michelsen said after finally converting his seventh match point.

"I've put in a lot of hard work and it's paying off a little bit. I was wondering when it would happen and I guess it is starting to happen right now.

"I returned really well today," he added. "I played offensive tennis and executed really well."

Michelson will face fellow American Learner Tien for a place in the quarter-finals. Tien beat compatriot Reilly Opelka 7-6 (7/3), 6-3.