SAN FRANCISCO -- Only 19 years old, Michael Mmoh of the United States has played in the main draw of the last two Grand Slam tournaments.
He has won a Challenger singles title.
He has beaten past or future top-100 players.
But until Wednesday, he had not defeated anyone currently in the top 100.
Taking the latest step in his promising career, Mmoh upset third-seeded Mikhail Kukushkin of Kazakhstan 6-3, 6-3 to reach the quarterfinals of the $100,000 KPSF Open indoors at the Bay Club SF Tennis Center.
Kukushkin is 10 years older than Mmoh and ranked 100 places higher at No. 93.
"This is my first win where the guy was actually top 100 when I played him, so I'm very happy to come away with the win, especially 3 and 3," Mmoh said. "I think that sends a message and gives me a lot of confidence going on to the rest of the year."
Mmoh kept balls in play with his breathtaking athleticism, which has been compared to that of ninth-ranked Gael Monfils, until Kukushkin made an error. Mmoh, powerfully built at 6-foot-1 (1.85 meters), often leaps into his shots and almost does the splits retrieving balls with his two-handed backhand.
Mmoh, who was born in Saudi Arabia to Nigeria native and ex-journeyman pro Tony Mmoh and an Irish mother who also holds Australian citizenship, acknowledged that defense "has always been a key to my success. I'm really athletic. It's very tough to get the ball by me. I'm going to make you play. You're going to have to hit maybe five winners to win the point.
"Right now, I'm just working on being more aggressive because I've always had the athleticism. I've always had the ability to get balls back, balls that 98 percent of people might not be able to get. I need to become a more complete player, and I think today I was hitting the ball. I was going after it. It was a good balance."
While crediting Mmoh, Kukushkin said he wasn't himself.
"Mmoh played really well today," conceded Kukushkin, a slender 6-foot (1.83-meter) Russia native. "He's a young guy, he was serving well, moving well, playing a lot of good balls and playing aggressive. He deserved to win. I was not in my best shape today and was not able to show my aggressive play."
After reaching a career-high No. 46 in October 2015, the right-handed Kukushkin was plagued by a right elbow injury throughout 2016 and tore a knee ligament at Wimbledon last year that did not require surgery. From last October until last week's $125,000 Dallas Challenger, he failed to win a match in six tournaments.
"Still coming into this season, I didn't feel 100 percent," Kukushkin moaned, "so that's why I'm kind of struggling right now."
Also Wednesday, the last three NCAA singles champions lost.
Wild card Ryan Shane (2014, Virginia) fell to another 19-year-old U.S. sensation, top-seeded Frances Tiafoe, 7-6 (4), 6-7 (4), 6-2 in an all-Washington, D.C.-area second-round matchup. The 6-foot-4 (1.93-meter) Shane, 22, was born and resides in Falls Church, Va. Tiafoe, ranked No. 94, was born in Hyattsville, Md., to emigrants from Sierra Leone but trains in Orlando, Fla.
Diminutive Marcos Giron (2015, UCLA) of Thousand Oaks in the Los Angeles area lost to fellow wild card Ramkumar Ramanathan of India 2-6, 6-4, 6-4 in the first round.
And Mackenzie McDonald (2016, UCLA), who grew up in Piedmont across the bay from San Francisco, fell to eighth-seeded Denis Kudla of Arlington, Va., in the Washington, D.C., area and Tampa, Fla., 7-6 (5), 4-6, 6-2 in a second-round battle of undersized players.
Kudla, 24, improved to 2-1 against McDonald, 21. All three matches have gone to three sets, and all have occurred in Northern California.
"For sure, he'll be a top-100 player -- guarantee it -- so I knew it would be tough," said Kudla, who's ranked No. 127 after reaching a career-high No. 53 last May. "Both of us are playing great against each other. He got me last time, so I knew what to expect, I expected an extremely high level, and he brought it.
"I made some improvements in my game the last couple months, since the last time we played. I think it definitely showed out there. I applied more pressure instead of the other way around and was dictating more."
Kudla, who seeks his third Challenger semifinal of the year, elaborated on his improvements.
"Just taking time away and hitting the ball bigger, being less predictable, tightening up on errors and consistency but not being a pusher," he said. "Being aggressive and taking the initiative, and I'm being successful at it right now."
Pounding his groundstrokes into the corners, Kudla reeled off the last four games of the deciding set.
"He played more aggressive for sure compared to last time," said the 266th-ranked McDonald, who turned pro last June after his junior year. "Maybe it was the indoor courts, but he's playing really well right now. Last time, I feel like he was just making a ton of balls and I was dictating a lot. But tonight, he was trying to dictate, too, so it made it tough."
If 6-foot-11 (2.11-meter) Reilly Opelka, 19, upsets seventh-seeded Vasek Pospisil of Canada, a distinct possibility, in tonight's featured singles match, Americans will have swept the quarterfinal berths in the top half of the draw. Tiafoe will face Pospisil or Opelka, and Kudla will play Mmoh.
In the bottom half, the only remaining American after the first round is qualifier Eric Quigley, and the only seeds left are No. 5 Henri Laaksonen of Switzerland and No. 6 Peter Polansky of Canada.
Here are the singles and doubles draws and today's schedule.