LONDON — Ernests Gulbis is a former top-10 player and Grand Slam semifinalist. A series of injuries pushed him down the rankings and kept him off the ATP Tour for long stretches, instead relegated to lower-tier Challenger tournaments.

Look at him now. Healthy and playing well, he's made it all the way to Wimbledon's fourth round for the first time in 11 appearances at the All England Club after upsetting No. 4 seed Alexander Zverev 7-6 (2), 4-6, 5-7, 6-3, 6-0 on Saturday.

The 29-year-old Gulbis is ranked only 138th and had to go through qualifying rounds to make it into Wimbledon's main draw. He'd never attempted that before and is now the first male qualifier to get to the fourth round in six years.

"I'm glad that I went through it," Gulbis said. "It makes me stronger mentally."

The victory over Zverev was his sixth in a row over the past two weeks.

It also allowed Gulbis to join 2017 French Open champion Jelena Ostapenko, who reached the women's fourth round with a victory earlier Saturday, to give Latvia two players in the round of 16 at a major tournament for the first time.

"It's great to see him doing this well, because I think he is such a talented player and he can play on a very high level," Ostapenko said about Gulbis. "I hope he's going to go even further in the tournament."

On Monday, Gulbis will face No. 24 seed Kei Nishikori of Japan, who was the runner-up at the 2014 U.S. Open.

The best Grand Slam showing for Gulbis was a run to the semifinals at the 2014 French Open, which included a win against Roger Federer. That was the year he reached his career-best ranking of No. 10.

But then came one injury after another and a slide to 589th a year ago because of a lack of matches.

He had won only one tour-level match this season before Wimbledon. Now he has his first three-match winning streak on tour since 2016.

And still, he can boast of a better Grand Slam performance on his resume than the 21-year-old Zverev, whose only major quarterfinal was last month at Roland Garros.

More than any edge in age or experience, though, what both men figured determined the outcome of this match was that Zverev ran out of energy. He came down with a stomach bug that affected him during his two-day, five-set victory over Taylor Fritz of the U.S. in the second round and felt tired down the stretch against Gulbis.

"It felt like somebody just unplugged me in the middle of the fourth set," Zverev said. "There was no going back."

___

Follow Howard Fendrich on Twitter at http://twitter.com/HowardFendrich

___

More AP tennis coverage: https://www.apnews.com/tag/apf-Tennis