Tyrrell Hatton (64) was fourth, while four-time major victor Rory McIlroy was among five players at 65.
"Conditions were tougher than they had been the last three days".
But such form deserves results.
In flawless scoring conditions at the Emirates Golf Club, the 42-year-old Donaldson took a one-shot lead over Anthony Wall and David Horsey.
The Chinese reacted with supreme composure in the clutch situation, going on to birdie four of his last six holes, including sinking a 25-foot putt on the 15th to draw level with McIlroy.McIlroy, a two-time victor in Dubai, went a long way right off the 16th tee and his failure to save par gave Li the outright lead.
Haotong Li believes he produced some of the shots of his life to register the biggest win of his career to date at the Omega Dubai Desert Classic.
The 24-year-old's first round ended poorly with a bogey, double bogey finish at the eighth and ninth.
The Pinner golfer started his "back" nine with a par four at the first before play was cut short for the day.
Li, who birdied four of the last six holes, said: "It's incredible, I'm so happy". "Then the 3-putt on 13, those were the two key holes of the tournament, really, even though there was a bad tee shot on 16".
"I just didn't realise I could make that putt on 15".
"But I tried until the very end".
The world No 60 bounced back from a bogey at the 12th to birdie the next and holed a putt from off the 15th green to close the gap on McIlroy, before securing victory with back-to-back gains over the final two holes. I made him win it in the end, which was, you know, all I could do, and he played very well on the way in, birdieing three of the last four.
He will also become the first man from China to break into the world's top 50, having previously made headlines in June 2017 when he threw his putter in a lake at the French Open, only for it to be fished out by his mother.
"The competitor in me is very disappointed right now", said the 28-year-old, victor here in 2009 and 2015. "I think most of the time, only two or one behind, so I just want to keep pulling myself in a position and just don't go too far away". "I'm looking forward to my first Masters".