Pete Banks, the first Yes guitarist, was drinking at the bar of the Speakeasy in London on April 9th. 1969, our first gig, when Crimson began playing. His drink never left the bar. Two days later the young Bill Bruford walked home to Fulham at five in the morning from the Strand Lyceum, raving about the group he had just seen. -- Robert Fripp [Link]
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Q: In May '69, King Crimson played their 'Marquee debut' and apparently their show provoked a big impact on Yes. Can you remember that particular show?
A: It wasn't at The Marquee, it was at the Speakeasy (April, 9th, 1969). I think their first ever gig was at the Speakeasy.
Q: Yes, thats right.
A: And I know I was there and Bill (Bruford) was there. I don't think Chris (Squire) was there. And I remember standing at the bar of the Speakeasy, which was a tiny little club, I mean, it was basically a restaurant with a bar and a tiny, tiny stage [....] I think that was the very first gig King Crimson ever did, certainly the first night I saw them, and it was absolutely amazing! And I was standing at the bar with a drink and I never touch my drink throughout the whole set, I just stood there in total amazement. Bill (Bruford) was standing next to me, I think, and we just stood there kind of open mouthed and...I think all of us were there. And we just watched them and immediately we were just amazed about how tight they were, and how good they were, and how good the composition was, and how original they sounded, and we just realized immediately... We thought that we were the best band around, and we probably were. We were pretty confident, as far as London bands went, that Yes were the one. We thought that nobody else was better than us. Crimson tore all that apart. I think we actually said that night we needed to rehearse a lot more.

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Peter Banks's diary records two Marquee shows on this date