Li Na safely through in Madrid
Li Na stretches to play a backhand against Serena Williams in the final of the Sony Open at Crandon Park Tennis Cente on March 29, 2014 in Key Biscayne, Florida - by Clive Brunskill
The world number two raced through the opening set, but was forced to save two set points in the second before closing out the match in a tie-break.
"I knew it would never be an easy match and even when I won the first set, she was really fighting a lot in the second," said the Chinese star.
Next up for Li is a meeting with compatriot Jie Zheng in round two.
"We are from the same country, we know each other well so I'm sure it will be tough," she added.
Number three seed Agnieszka Radwanska also sealed her place in the last 32 with a 7-6 (7/3), 6-2 win over Australian Open semi-finalist Eugenie Bouchard.
The Canadian put up a strong fight in a tightly contested opening set, but Radwanska's variety of shot-making allowed her to comfortably take the tie-break.
"Of course I think the tiebreak was the biggest key in that match," admitted Radwanska.
"I made a couple good shots in the tiebreak and managed to win that. I think that gave me more confidence in the second set and it was a little bit easier."
The Pole took complete control in the second with an early double break and served the match out to set up a second round clash with Russia's Svetlana Kuznetsova.
Australian Open finalist Dominika Cibulkova of Slovakia became the first seed to fall on Monday, as she was comprehensively beaten 6-3, 6-0 by 2011 US Open champion Sam Stosur of Australia.
It was smooth sailing for the seeds on the men's side as Kei Nishikori, John Isner and Jo-Wilfried Tsonga all secured their passage into round two.
Japan's Nishikori kept his recent fine form going after winning the Barcelona Open last week with a 6-4, 6-4 win over Croatian Ivan Dodig to set up a meeting with Spaniard Guillermo Garcia-Lopez in the second round.
Ninth seed Isner was characteristically solid on serve as the American wasn't broken in his 7-6, (10/8), 6-4 win over Russia's Teymuraz Gabashvili.
Tsonga faced a stiffer test against fellow Frenchman Edouard Roger-Vasselin, but also came through in three sets 6-3, 4-6, 6-3.