The 27-year-old Indian, who won a silver medal at the last edition at Jakarta, dug deep into her reservoir to eke out a 21-19 18-21 21-15 win over the World No. 31 Gilmour in a match that lasted an hour and 14 minutes. Saina thus joined compatriot and two-time bronze medallist P V Sindhu into the semifinals.
The London Olympics bronze medallist will next play seventh seed Nozomi Okuhara of Japan, who knocked out two-time defending champion Spain's Carolina Marin 21-18 14-21 21-15 in another match.
Saina had opened up a 6-2 lead but Gilmour managed to claw back. It was a jump smash that helped her to draw parity at 8-8 and she soon moved into the break with a 11-8 lead after setting it up beautifully with her angled strokes. Saina seemed to lack the length in her returns which could put her rival out of position and she looked frustrated as Gilmour was dictating the turns with her repertoire of strokes.
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Saina managed to claw back at 16-17 when Gilmour produced an onrushing return to break the momentum. The scot made some errors, hitting the shuttle out to allow Saina level par at 18-18. A good serve gave Saina another point as she moved to game point when her rival made an error in judgement at backline and the experienced Indian sealed it when the Scot found the net.
However, Gilmour once again had the bragging rights at the break as she took the 11-9 lead. The Scot tried to draw Saina forward and follow it up with a fast-paced return and it helped as she lead 17-11.
The Scot constructed rallies well but she made many unforced errors which helped Saina make it 16-19. Just then Gilmour produced a jump smash to move to game point but experienced Saina played the right net shot to gain another point. Gilmour found the net again but another tricky play during the rally helped her to roar back into contest when the Indian hit the net.
Gilmour produced another towering cross smash and then triggered one at Saina's body to gain a couple of points. But Saina made some good judgements with the shuttler to move to 14-8. The Indian gave her everything to dominate the rallies and mixed her strokes. But Gilmour was always lurking behind with her equally lethal deceptions and strokeplay.
A fighting Gilmour refused to give up and saved a couple of match points but the Indian sealed the issue when Gilmour found the net.