Tallon Griekspoor has upset top-seed Alexander Zverev in a marathon second-round clash at Indian Wells, where there were mixed results for Australians.

Griekspoor collapsed on the court after converting his sixth match point to end an absorbing three-hour, seven-minute affair, eventually prevailing 4-6 7-6 (7-5) 7-6 (7-4).

While it was a great day for the Dutchman, there were mixed results for Australian hopefuls in the California desert.

Alexei Popyrin, the No.26 seed, started his campaign in fine style, beating up-and-coming Belgian Zizou Bergs 7-6 (7-4) 7-5.

The win moves Popyrin into the third round at Indian Wells for the first time in five main-draw appearances.

Unfortunately, Popyrin's compatriot Chris O'Connell fell to resurgent Italian Matteo Berrettini, the former world No.6 winning 6-2 7-6 (7-2).

Fellow Australian Alex de Minaur, the No.9 seed, kicks off his campaign against another Belgian, veteran David Goffin.

Rinky Hijikata and Adam Walton, who won their first-round matches earlier in the week, take on seeds Brandon Nakashima and Denis Shapovalov respectively.

Meanwhile, Griekspoor was delighted by his marathon win over Zverev, achieved under sunny skies on Stadium One.

"I'm incredibly proud. It was such a mental battle to beat him," Griekspoor said after beating the German, who had dominated their match-ups coming into the contest.

"I beat him once but then lost the next six times, so it was such a mental thing. Happy to finally get it out of the way."

Zverev served for the match in the second set but was unable to get over the line, and Griekspoor edged him in the second-set tiebreaker to force a decider.

Griekspoor failed to convert five match points with the match on his racquet at 6-5 in the third, but regrouped to win the deciding tiebreak.

It was Griekspoor's first win over a top-five opponent in 19 attempts. He faces big-serving Frenchman Giovanni Mpetshi Perricard in the third round.

Zverev has struggled with his form since losing to world No.1 Jannik Sinner in the Australian Open final in January.

"I'm not playing a level that I want to play, definitely not playing anywhere near what I played in Australia,'' he said.

In another major upset, fourth seed Casper Ruud fell to American Marcos Giron 7-6 (7-4) 3-6 6-2.

Giron was ecstatic after shocking the Norwegian, leaping into the air and shouting as the crowd erupted after he reached the third round of the tournament for a second time.

"To do it here at my favourite tournament is special,'' Giron said. "It was the tournament I came to when I was younger and it was an amazing atmosphere.''

Popular 10th-seed Tommy Paul eased past fellow American Tristan Boyer 6-3 6-1. Paul, who reached the semi-finals here last year, was among the players who fell ill with a stomach bug at the Mexican Open in Acapulco last month, but said he had fully recovered.

The American next faces Britain's Cameron Norrie, who beat 23rd seed Jiri Lehecka 3-6 6-4 7-5.

Denmark's Holger Rune, seeded 12th, will face 18th-seeded Frenchman Ugo Humbert in the third round after they both enjoyed straight-sets victories.

Russia's Daniil Medvedev, finalist in Indian Wells for the past two years, beat China's Yunchaokete Bu in straight sets.