Against all odds, we could have the tennis version of Halley’s Comet, an unseeded player winning the U.S. Open. Juan Martin del Potro, the world’s 142nd-ranked player, yesterday proceeded to the quarterfinals when his opponent, 8th-seeded Dominic Thiem of Austria, retired five games into the second set.

Del Potro still is three victories from lifting the tournament trophy. That remains highly unreasonable, but it has happened before. In 2009, unseeded Kim Clijsters won the Open’s women’s title. In 1994, unseeded Andre Agassi was the men’s champion.

And this is the 50th anniversary of unseeded Fred Stolle winning the U.S. national championship title, two years before the start of the open era.

With del Potro, we could have another tennis reincarnation. Before enduring years of wrist problems which led to four operations, del Potro was the 2009 Open champ. That, of course, was the year that Clijsters—like del Potro this year—was a wild-card entry.

Want more karma? Clijsters—like del Potro now and Agassi in ’94—had previously won a major tournament. For Clijsters, that was the 2005 Open, before a two-year sabbatical to start a family. Agassi had won Wimbledon ’92 before he, like del Potro was sidelined by wrist surgery. His ranking subsequently dropped to No. 20 for the ’94 Open, when only 16 players were seeded. (The move to 32 seeds came in 2001.)

Yesterday, after the 22-year-old Thiem—down 3-6, 2-3 to del Potro—withdrew with a right knee injury, del Potro was asked if he was felt any echoes of his progress through the draw in 2009.

“I don’t remember,” he said. “I only remember the final against Roger. And he’s not here.”

With Roger Federer absent while he rehabilitates a surgically repaired knee, and the way action in the men’s draw is playing out—high seeds Rafael Nadal (No. 4), Milos Raonic (No. 5) and Marin Cilic (No. 7) all have been eliminated—del Potro has emerged as a main attraction.

“Of course I have great memories from 2009, but now my person is completely different,” del Potro said. “My life is different. I have a different game. I’m getting older . Everything is like new to me. It’s like a new career.”

He has beaten the 69th-ranked player, fellow Argentine Diego Schwartzman, 19th-seeded American Steve Johnson and 11th-seeded David Ferrer of Spain, and was fully in control yesterday when Thiem quit.

Not since 174th-ranked, 39-year-old Jimmy Connors’ memorable run to the semifinals 25 years ago has a player ranked as low as del Potro made the Open quarterfinals.

“I’m enjoying even more when I get into the courts than years ago,” del Potro said. “I just want to play tennis in front of the big crowds. I’m so excited to still be winning matches and maybe be in the top position in the future.

“But I don’t care about that at the moment. I’m here. I’m playing free. I don’t have any problems with my wrist.”