In what turned out to be the match of the day, Ramkumar had to dig deep into his reserves before eking out a 6-7 (5) 7-6(5), 7-6(9) win in the opening round of the USD 50,000 hard court event.
In the two-hour-56 minutes match, Ramkumar could convert only one of the 12 breakpoints as the Australian, ranked 322, came up with a stiff resistance.
Third seed Yuki, who won the title here in 2015 and broke into top-100 for the first time, fought off Japanese qualifier Kaichi Uchida 6-2 7-5.
Yuki trailed 1-4 in the second set but recovered swiftly to turn the match in his favour.
Earlier, Sumit Nagal defeated Austria's Lucas Miedler 7- 6(1) 6-0 and 'lucky loser' N Prashant made best use of the opportunity to end the campaign of wild card Aryan Goveas 6-3 6-4 in an all-Indian clash.
Top seed Slovenian Blaz Kavcic got through his round against Geoffey Blancaneaux (France) 6-4 6-4 on an outside court while Spanish second seed A Menendez Maceiras showed qualifier Borna Gojo (Croatia) the door with a 6-2 6-3 win.
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