Nagal got the better of Istomin 6-4 6-7(6) 6-4 in two hours and 34 minutes on court 10 at the Ariake Tennis Centre to set up a second round with world number two Daniil Medvedev.
Medvedev extended his grass-court winning streak to seven matches with the 6-7(3), 3-6, 6-3, 6-3, 6-2 victory, a three hour, 36-minute marathon of twists and turns
"It's probably one of the sweetest victories in my career because I was cramping like hell in the third set," Medvedev said, Xinhua news reports.
"I just came actually came from another MRI that I've done this morning and the damage is bigger than what it was when it happened when I did the first MRI after the third round," Djokovic told reporters.
The win moves world No 1 Djokovic, 33, within two of Swiss great Roger Federer and Spain's Rafael Nadal's record total of 20 major victories
Medvedev will move to the world No. 2 spot if he wins the final.
The Russian keeps date with Djokovic in Sunday summit clash
Medvedev beat the seventh seed Rublev 7-5, 6-3, 6-2 and will now face either world No.5 Stefanos Tsitsipas or 20-time Grand Slam champion Rafael Nadal in the semi-finals.
"I always said before this tournament that it would be an amazing story if, here in London, where the tournament was for (12) years, that the first champion would be Russian and the last champion would be Russian, too," Medvedev said
Medvedev, who lost all three group matches in last year's season finale, had claimed his first victory in the tournament two days ago by beating 2018 champion Alexander Zverev in straight sets, reports Xinhua news agency.
Medvedev prevailed 6-3, 6-4, while Djokovic cruised past Finals debutant Diego Schwartzman 6-3, 6-2 in an empty O2 Arena on Monday
The men's tennis season finale is going to be held in London from November 15 to 22 and the top eight singles and doubles players learned their groups for the tournament on Thursday, reports Xinhua news agency.
Medvedev defeated Zverev 5-7, 6-4, 6-1 that also ended the German's 12-match winning streak on Sunday. The Russian, who was contesting his first final of 2020, had to survive some outrageous rallies
The Russian was on top form in a high-quality encounter, winning 5-7, 6-4, 6-1 to secure his first trophy of the season and the eighth of his career.
German fourth seed dominated Nadal at Bercy Arena to win their semi-final 6-4, 7-5 and claim his second straight victory over the 20-time Grand Slam champion
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