(Photo credit: John E. Sokolowski-Imagn Images)

Alex Michelsen pulled off a comeback for a notable upset in the third round of the National Bank Open at Toronto on Thursday, but getting to the finish was no easy feat.

The 26th-seeded U.S. player emerged with a 3-6, 7-6 (4), 6-4 win over third-seeded Lorenzo Musetti of Italy, closing it out on his seventh match point.

Up 5-3 in the third set, Michelsen squandered two match points on Musetti's serve, then needed five more match points on his own serve (while turning aside two break points for Musetti) before finally ending the 2-hour, 32-minute contest.

The ending reflected a match in which Michelsen, 20, took advantage of just 2 of 9 break-point opportunities while Musetti, 23, was 2 of 12 on break-point chances.

Michelsen leaned on his groundstrokes for the win.

'I am a big guy, I have got to play offensive tennis,' he said. 'Especially today, you can't give a guy like Lorenzo time to bully you around from the baseline, so I was trying to bully him around and I think in the second and third sets I did a really good job. Taking shots early. I was missing a couple but you can't make them all and I was doing a really good job executing.'

Michelsen moves on to the fourth round of an ATP 1000 event for the first time. Next, he will meet fellow U.S. player Learner Tien, who beat countryman Reilly Opelka 7-6 (3), 6-3 on Thursday.

'The first time in the fourth round feels really good,' Michelsen said. 'I have put a lot of hard work in and it is finally starting to pay off a little bit. I was wondering when it would happen and I guess it is starting to happen right now.'

The day's other matches all saw the higher-seeded players come out on top.

No. 1 seed Alexander Zverev of Germany rallied for a 6-7 (5), 6-3, 6-2 win over No. 32 Matteo Arnaldi of Italy. No. 5 Holger Rune of Denmark downed No. 29 Alexandre Muller of France 6-2, 6-4, and No. 8 Casper Ruud of Norway beat No. 30 Nuno Borges of Portugal 7-5, 6-4.

No. 11 Karen Khachanov of Russia defeated the United States' Emilio Nava 6-7 (6), 6-4, 6-1, and No. 14 Francisco Cerundolo won an all-Argentine matchup with Tomas Martin Etcheverry, 6-3, 6-4.

In the final match of the night, No. 10 Daniil Medvedev of Russia squared off with No. 18 Alexei Popyrin of Australia.

--Field Level Media