diff --git a/src/models/event.ts b/src/models/event.ts
index 317377b1a..fcbdba3fa 100644
--- a/src/models/event.ts
+++ b/src/models/event.ts
@@ -420,6 +420,11 @@ export class MatrixEvent extends EventEmitter {
             || this.thread instanceof Thread;
     }
 
+    public get parentEventId(): string {
+        return this.replyEventId
+            || this.getWireContent()["m.relates_to"]?.event_id;
+    }
+
     /**
      * Get the previous event content JSON. This will only return something for
      * state events which exist in the timeline.
diff --git a/src/models/room.ts b/src/models/room.ts
index ac80d9ceb..41d417417 100644
--- a/src/models/room.ts
+++ b/src/models/room.ts
@@ -1305,8 +1305,7 @@ export class Room extends EventEmitter {
                 this.handleRemoteEcho(event, existingEvent);
             }
         }
-
-        let thread = this.findEventById(event.replyEventId)?.getThread();
+        let thread = this.findEventById(event.parentEventId)?.getThread();
         if (thread) {
             thread.addEvent(event);
         } else {
diff --git a/src/models/thread.ts b/src/models/thread.ts
index 844fe2210..0415c554a 100644
--- a/src/models/thread.ts
+++ b/src/models/thread.ts
@@ -149,6 +149,10 @@ export class Thread extends EventEmitter {
         return this.findEventById(this.root);
     }
 
+    public get roomId(): string {
+        return this.rootEvent.getRoomId();
+    }
+
     /**
      * The number of messages in the thread
      */
diff --git a/src/sync.ts b/src/sync.ts
index df9842132..0c5a81638 100644
--- a/src/sync.ts
+++ b/src/sync.ts
@@ -316,8 +316,18 @@ export class SyncApi {
         if (this.opts.experimentalThreadSupport) {
             return events.reduce((memo, event: MatrixEvent) => {
                 const room = this.client.getRoom(event.getRoomId());
-                const eventInThread = this.shouldLiveInThreadTimeline(event, room);
-                memo[eventInThread ? 1 : 0].push(event);
+                // An event should live in the thread timeline if
+                // - It's a reply in thread event
+                // - It's related to a reply in thread event
+                let shouldLiveInThreadTimeline = event.replyInThread;
+                if (!shouldLiveInThreadTimeline) {
+                    const parentEventId = event.getWireContent()["m.relates_to"]?.event_id;
+                    const parentEvent = room?.findEventById(parentEventId) || events.find((mxEv: MatrixEvent) => {
+                        return mxEv.getId() === parentEventId;
+                    });
+                    shouldLiveInThreadTimeline = parentEvent?.replyInThread;
+                }
+                memo[shouldLiveInThreadTimeline ? 1 : 0].push(event);
                 return memo;
             }, [[], []]);
         } else {
@@ -330,50 +340,6 @@ export class SyncApi {
         }
     }
 
-    /**
-     * @experimental
-     * An event should live in a thread if it's a threaded reply
-     * Or if it's related to an event that is a threaded reply
-     * Things like annotations, redactions, ...
-     */
-    private shouldLiveInThreadTimeline(event: MatrixEvent, room: Room): boolean {
-        let isThreadReply = event.replyInThread;
-
-        // If the message is sent from a client that does not support threads
-        // The relation body will not contain `UNSTABLE_ELEMENT_REPLY_IN_THREAD`
-        // We go up the reply chain looking for an event that has this property
-        // If we find one, we then decide to render this reply inside the thread
-        if (!isThreadReply && event.replyEventId) {
-            isThreadReply = this.ancestorInThread(event, room);
-        }
-
-        let isRelatedToThreadReply = false;
-        const relationEventId = event.getRelation()?.event_id;
-        if (relationEventId) {
-            const relatedEvent = room.findEventById(relationEventId);
-            isRelatedToThreadReply = relatedEvent.replyInThread;
-        }
-
-        return isThreadReply || isRelatedToThreadReply;
-    }
-
-    /**
-     * @experimental
-     * Checks whether an event up in the reply chain has
-     * the `UNSTABLE_ELEMENT_REPLY_IN_THREAD` property in its
-     * relations body
-     */
-    private ancestorInThread(event: MatrixEvent, room: Room): boolean {
-        const relatedEvent = room.findEventById(event.replyEventId);
-        if (relatedEvent.replyInThread) {
-            return true;
-        } else if (relatedEvent.replyEventId) {
-            return this.ancestorInThread(relatedEvent, room);
-        } else {
-            return false;
-        }
-    }
-
     /**
      * Peek into a room. This will result in the room in question being synced so it
      * is accessible via getRooms(). Live updates for the room will be provided.